Learn Develop Data Engineering: 2016

PL/SQL stands for Procedural Language extension of SQL.PL/SQL is a combination of SQL along with the procedural features of programming languages. It was developed by Oracle Corporation in the early 90’s to enhance the capabilities of SQL.

The PL/SQL Engine: Oracle uses a PL/SQL engine to processes the PL/SQL statements. A PL/SQL language code can be stored in the client system (client-side) or in the database (server-side).

A Simple PL/SQL Block:

Each PL/SQL program consists of SQL and PL/SQL statements which from a PL/SQL block.

PL/SQL Block consists of three sections:

The Declaration section (optional).

The Execution section (mandatory).

The Exception Handling (or Error) section (optional).

The Declaration section of a PL/SQL Block starts with the reserved keyword DECLARE. This section is optional and is used to declare any placeholders like variables, constants, records and cursors, which are used to manipulate data in the execution section. Placeholders may be any of Variables, Constants and Records, which stores data temporarily. Cursors are also declared in this section.
The Execution section of a PL/SQL Block starts with the reserved keyword BEGIN and ends with END. This is a mandatory section and is the section where the program logic is written to perform any task. The programmatic constructs like loops, conditional statement and SQL statements form the part of execution section.
The Exception section of a PL/SQL Block starts with the reserved keyword EXCEPTION. This section is optional. Any errors in the program can be handled in this section, so that the PL/SQL Blocks terminates gracefully. If the PL/SQL Block contains exceptions that cannot be handled, the Block terminates abruptly with errors.
Every statement in the above three sections must end with a semicolon; PL/SQL blocks can be nested within other PL/SQL blocks. Comments can be used to document code.

DECLARE
     Variable declaration
BEGIN
     Program Execution
EXCEPTION
     Exception handling
END;

These are the Advantages of PL/SQL

Block Structures: PL SQL consists of blocks of code, which can be nested within each other. Each block forms a unit of a task or a logical module. PL/SQL Blocks can be stored in the database and reused.
Procedural Language Capability: PL SQL consists of procedural language constructs such as conditional statements (if else statements) and loops like (FOR loops).
Better Performance: PL SQL engine processes multiple SQL statements simultaneously as a single block, thereby reducing network traffic.
Error Handling: PL/SQL handles errors or exceptions effectively during the execution of a PL/SQL program. Once an exception is caught, specific actions can be taken depending upon the type of the exception or it can be displayed to the user with a message.

PL/SQL Placeholders

Placeholders are temporary storage area. PL/SQL Placeholders can be any of Variables, Constants and Records. Oracle defines placeholders to store data temporarily, which are used to manipulate data during the execution of a PL SQL block.

Define PL/SQL Placeholders

Depending on the kind of data you want to store, you can define placeholders with a name and a datatype. Few of the datatypes used to define placeholders are as given below.
Number (n,m) , Char (n) , Varchar2 (n) , Date , Long , Long raw, Raw, Blob, Clob, Nclob, Bfile

PL/SQL Variables

These are placeholders that store the values that can change through the PL/SQL Block.

General Syntax to declare a variable is

variable_name datatype [NOT NULL := value ];

variable_name is the name of the variable.
datatype is a valid PL/SQL datatype.
NOT NULL is an optional specification on the variable.
value or DEFAULT value is also an optional specification, where you can initialize a variable.
Each variable declaration is a separate statement and must be terminated by a semicolon.

For example, if you want to store the current salary of an employee, you can use a variable.

DECLARE

salary number (6);

* “salary” is a variable of datatype number and of length 6.

When a variable is specified as NOT NULL, you must initialize the variable when it is declared.

The General Syntax is:

SELECT column_name

INTO variable_name

FROM table_name

[WHERE condition];

Local variables - These are declared in a inner block and cannot be referenced by outside Blocks.
Global variables - These are declared in a outer block and can be referenced by its itself and by its inner blocks.

DECLARE

var_num1 number;

var_num2 number;

BEGIN

var_num1 := 100;

var_num2 := 200;

DECLARE

var_mult number;

BEGIN

var_mult := var_num1 * var_num2;

END;

As the name implies a constant is a value used in a PL/SQL Block that remains unchanged throughout the program. A constant is a user-defined literal value. You can declare a constant and use it instead of actual value.

For example: If you want to write a program which will increase the salary of the employees by 25%, you can declare a constant and use it throughout the program. Next time when you want to increase the salary again you can change the value of the constant which will be easier than changing the actual value throughout the program.

General Syntax to declare a constant is:

constant_name CONSTANT datatype := VALUE;

constant_name is the name of the constant i.e. similar to a variable name.
The word CONSTANT is a reserved word and ensures that the value does not change.
VALUE - It is a value which must be assigned to a constant when it is declared. You cannot assign a value later.

For example, to declare salary_increase, you can write code as follows:

DECLARE

salary_increase CONSTANT number (3) := 10;

You must assign a value to a constant at the time you declare it. If you do not assign a value to a constant while declaring it and try to assign a value in the execution section, you will get a error. If you execute the below Pl/SQL block you will get error.

DECLARE

salary_increase CONSTANT number(3);

BEGIN

salary_increase := 100;

dbms_output.put_line (salary_increase);

END;

What are records?

Records are another type of datatypes which oracle allows to be defined as a placeholder. Records are composite datatypes, which means it is a combination of different scalar datatypes like char, varchar, number etc. Each scalar data types in the record holds a value. A record can be visualized as a row of data. It can contain all the contents of a row.

Declaring a record:To declare a record, you must first define a composite datatype; then declare a record for that type.The General Syntax to define a composite datatype is:

TYPE record_type_name IS RECORD

(first_col_name column_datatype,

second_col_name column_datatype, ...);

record_type_name – it is the name of the composite type you want to define.
first_col_name, second_col_name, etc.,- it is the names the fields/columns within the record.
column_datatype defines the scalar datatype of the fields.

There are different ways you can declare the datatype of the fields.

1) You can declare the field in the same way as you declare the fieds while creating the table.
2) If a field is based on a column from database table, you can define the field_type as follows:

col_name table_name.column_name%type;

The General Syntax to declare a record of a uer-defined datatype is:

record_name record_type_name;

DECLARE

TYPE employee_type IS RECORD

(employee_id number(5),

employee_first_name varchar2(25),

employee_last_name employee.last_name%type,

employee_dept employee.dept%type);

employee_salary employee.salary%type;

employee_rec employee_type;

record_name table_name%ROWTYPE;

DECLARE

employee_rec employee%ROWTYPE;

1) You do not need to explicitly declare variables for all the columns in a table.
2) If you alter the column specification in the database table, you do not need to update the code.
1) When u create a record as a ROWTYPE, fields will be created for all the columns in the table and memory will be used to create the datatype for all the fields. So use ROWTYPE only when you are using all the columns of the table in the program.
The following table consolidates the different ways in which you can define and declare a pl/sql record.

Syntax	Usage
TYPE record_type_name IS RECORD (column_name1 datatype, column_name2 datatype, ...);	Define a composite datatype, where each field is scalar.
col_name table_name.column_name%type;	Dynamically define the datatype of a column based on a database column.
record_name record_type_name;	Declare a record based on a user-defined type.
record_name table_name%ROWTYPE;	Dynamically declare a record based on an entire row of a table. Each column in the table corresponds to a field in the record.

The General Syntax to assign a value to a column within a record direclty is:

record_name.col_name := value;

record_name.column_name := value;

SELECT col1, col2

INTO record_name.col_name1, record_name.col_name2

FROM table_name

[WHERE clause];

SELECT * INTO record_name

FROM table_name

[WHERE clause];

The General Syntax to retrieve a value from a specific field into another variable is:

var_name := record_name.col_name;

Syntax	Usage
record_name.col_name := value;	To directly assign a value to a specific column of a record.
record_name.column_name := value;	To directly assign a value to a specific column of a record, if the record is declared using %ROWTYPE.
SELECT col1, col2 INTO record_name.col_name1, record_name.col_name2 FROM table_name [WHERE clause];	To assign values to each field of a record from the database table.
SELECT * INTO record_name FROM table_name [WHERE clause];	To assign a value to all fields in the record from a database table.
variable_name := record_name.col_name;	To get a value from a record column and assigning it to a variable.

Conditional Statements in PL/SQL

As the name implies, PL/SQL supports programming language features like conditional statements, iterative statements.

The programming constructs are similar to how you use in programming languages like Java and C++.

In this section I will provide you syntax of how to use conditional statements in PL/SQL programming.

Conditional Statements in PL/SQL

IF THEN ELSE STATEMENT

1)

IF condition

THEN

 statement 1;

ELSE

 statement 2;

END IF;

2)

IF condition 1

THEN

 statement 1;

 statement 2;

ELSIF condtion2 THEN

 statement 3;

ELSE

 statement 4;

END IF

3)
IF condition 1
THEN
statement 1;
statement 2;
ELSIF condtion2 THEN
statement 3;
ELSE
statement 4;
END IF;

4)
IF condition1 THEN
ELSE
IF condition2 THEN
statement1;
END IF;
ELSIF condition3 THEN
statement2;
END IF;

Iterative control Statements are used when we want to repeat the execution of one or more statements for specified number of times.

There are three types of loops in PL/SQL:

• Simple Loop
• While Loop
• For Loop

1) Simple Loop

A Simple Loop is used when a set of statements is to be executed at least once before the loop terminates. An EXIT condition must be specified in the loop, otherwise the loop will get into an infinite number of iterations. When the EXIT condition is satisfied the process exits from the loop.

General Syntax to write a Simple Loop is:

LOOP

statements;

EXIT;

{or EXIT WHEN condition;}

END LOOP;

These are the important steps to be followed while using Simple Loop.

1) Initialize a variable before the loop body.
2) Increment the variable in the loop.
3) Use an EXIT WHEN statement to exit from the Loop. If you use a EXIT statement without WHEN condition, the statements in the loop is executed only once.

2) While Loop

A WHILE LOOP is used when a set of statements has to be executed as long as a condition is true. The condition is evaluated at the beginning of each iteration. The iteration continues until the condition becomes false.

The General Syntax to write a WHILE LOOP is:

WHILE <condition>

LOOP statements;

END LOOP;

Important steps to follow when executing a while loop:

1) Initialize a variable before the loop body.
2) Increment the variable in the loop.
3) EXIT WHEN statement and EXIT statements can be used in while loops but it's not done often.

3) FOR Loop

A FOR LOOP is used to execute a set of statements for a predetermined number of times. Iteration occurs between the start and end integer values given. The counter is always incremented by 1. The loop exits when the counter reaches the value of the end integer.

The General Syntax to write a FOR LOOP is:

FOR counter IN val1..val2

LOOP statements;

END LOOP;

val1 - Start integer value.
val2 - End integer value.

Important steps to follow when executing a while loop:

1) The counter variable is implicitly declared in the declaration section, so it's not necessary to declare it explicitly.
2) The counter variable is incremented by 1 and does not need to be incremented explicitly.
3) EXIT WHEN statement and EXIT statements can be used in FOR loops but it's not done often.

NOTE: The above Loops are explained with an example when dealing with Explicit Cursors.

What are Cursors?

A cursor is a temporary work area created in the system memory when a SQL statement is executed. A cursor contains information on a select statement and the rows of data accessed by it.

This temporary work area is used to store the data retrieved from the database, and manipulate this data. A cursor can hold more than one row, but can process only one row at a time. The set of rows the cursor holds is called the active set.

There are two types of cursors in PL/SQL:

Implicit cursors:

These are created by default when DML statements like, INSERT, UPDATE, and DELETE statements are executed. They are also created when a SELECT statement that returns just one row is executed.

Explicit cursors:

They must be created when you are executing a SELECT statement that returns more than one row. Even though the cursor stores multiple records, only one record can be processed at a time, which is called as current row. When you fetch a row the current row position moves to next row.

Both implicit and explicit cursors have the same functionality, but they differ in the way they are accessed.

Implicit Cursors: Application
When a SELECT... INTO statement is executed in a PL/SQL Block, implicit cursor attributes can be used to find out whether any row has been returned by the SELECT statement. PL/SQL returns an error when no data is selected.

Attributes	Return Value	Example
%FOUND	The return value is TRUE, if the DML statements like INSERT, DELETE and UPDATE affect at least one row and if SELECT ….INTO statement return at least one row.	SQL%FOUND
%FOUND	The return value is FALSE, if DML statements like INSERT, DELETE and UPDATE do not affect row and if SELECT….INTO statement do not return a row.	SQL%FOUND
%NOTFOUND	The return value is FALSE, if DML statements like INSERT, DELETE and UPDATE at least one row and if SELECT ….INTO statement return at least one row.	SQL%NOTFOUND
%NOTFOUND	The return value is TRUE, if a DML statement like INSERT, DELETE and UPDATE do not affect even one row and if SELECT ….INTO statement does not return a row.	SQL%NOTFOUND
%ROWCOUNT	Return the number of rows affected by the DML operations INSERT, DELETE, UPDATE, SELECT	SQL%ROWCOUNT

DECLARE var_rows number(5);
BEGIN
UPDATE employee
  SET salary = salary + 1000;
IF SQL%NOTFOUND THEN
    dbms_output.put_line('None of the salaries where updated');
ELSIF SQL%FOUND THEN
    var_rows := SQL%ROWCOUNT;
    dbms_output.put_line('Salaries for ' || var_rows || 'employees are updated');
END IF;
END;

Explicit Cursors

An explicit cursor is defined in the declaration section of the PL/SQL Block. It is created on a SELECT Statement which returns more than one row. We can provide a suitable name for the cursor.

General Syntax for creating a cursor is as given below:

CURSOR cursor_name IS select_statement;

cursor_name – A suitable name for the cursor.
select_statement – A select query which returns multiple rows.

How to use Explicit Cursor?

There are four steps in using an Explicit Cursor.

DECLARE the cursor in the declaration section.
OPEN the cursor in the Execution Section.
FETCH the data from cursor into PL/SQL variables or records in the Execution Section.
CLOSE the cursor in the Execution Section before you end the PL/SQL Block.

   DECLARE
   CURSOR emp_cur IS
   SELECT *
   FROM emp_tbl
   WHERE salary > 5000;
employees with salary greater than 5000. Here ‘emp_tbl’ in the table which contains records of all the employees.
Once the cursor is created in the declaration section we can access the cursor in the execution section of the PL/SQL program.
How to access an Explicit Cursor?
1) Open the cursor.
2) Fetch the records in the cursor one at a time.
3) Close the cursor.
OPEN cursor_name;
FETCH cursor_name INTO record_name;
FETCH cursor_name INTO variable_list;
CLOSE cursor_name;

If you are fetching a cursor to a PL/SQL Record, the record should have the same structure as the cursor.
If you are fetching a cursor to a list of variables, the variables should be listed in the same order in the fetch statement as the columns are present in the cursor.

DECLARE
    variables;
    records;
    create a cursor;
BEGIN
   OPEN cursor;
   FETCH cursor;
     process the records;
   CLOSE cursor;
END;
Explicit Cursor, Lets Look at the example below
Example 1:
DECLARE
emp_rec emp_tbl%rowtype;
CURSOR emp_cur IS
SELECT * FROM WHERE salary > 10;
BEGIN
OPEN emp_cur;
FETCH emp_cur INTO emp_rec;
dbms_output.put_line (emp_rec.first_name || ' ' || emp_rec.last_name);
CLOSE emp_cur;
END;
What are Explicit Cursor Attributes?
When does an error occur while accessing an explicit cursor?
b) When we try to fetch a cursor after the last operation.

Attributes	Return values	Example
%FOUND	TRUE, if fetch statement returns at least one row.	Cursor_name%FOUND
	FALSE, if fetch statement doesn’t return a row.
%NOTFOUND	TRUE, , if fetch statement doesn’t return a row.	Cursor_name%NOTFOUND
	FALSE, if fetch statement returns at least one row.
%ROWCOUNT	The number of rows fetched by the fetch statement	Cursor_name%ROWCOUNT
	If no row is returned, the PL/SQL statement returns an error.
%ISOPEN	TRUE, if the cursor is already open in the program	Cursor_name%ISNAME
	FALSE, if the cursor is not opened in the program.

Using Loops with Explicit Cursors:
Cursor with a Simple Loop:
DECLARE
CURSOR emp_cur IS
SELECT first_name, last_name, salary FROM emp_tbl;
emp_rec emp_cur%rowtype;
BEGIN
IF NOT sales_cur%ISOPEN THEN
OPEN sales_cur;
END IF;
LOOP
FETCH emp_cur INTO emp_rec;
EXIT WHEN emp_cur%NOTFOUND;
dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name
|| ' ' ||emp_cur.salary);
END LOOP;
END;
/
In line no 6, we are using the cursor attribute %ISOPEN to check if the cursor is open, if the condition is true the program does not open the cursor again, it directly moves to line no 9.
In line no 11, we are using the cursor attribute %NOTFOUND to check whether the fetch returned any row. If there is no rows found the program would exit, a condition which exists when you fetch the cursor after the last row, if there is a row found the program continues.
Cursor with a While Loop:
DECLARE
CURSOR emp_cur IS
SELECT first_name, last_name, salary FROM emp_tbl;
emp_rec emp_cur%rowtype;
BEGIN
IF NOT sales_cur%ISOPEN THEN
OPEN sales_cur;
END IF;
FETCH sales_cur INTO sales_rec;
WHILE sales_cur%FOUND THEN
LOOP
dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name
|| ' ' ||emp_cur.salary);
FETCH sales_cur INTO sales_rec;
END LOOP;
END;
/
Cursor with a FOR Loop:
FOR record_name IN cusror_name
LOOP
process the row...
END LOOP;
DECLARE
CURSOR emp_cur IS
SELECT first_name, last_name, salary FROM emp_tbl;
emp_rec emp_cur%rowtype;
BEGIN
FOR emp_rec in sales_cur
LOOP
dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name
|| ' ' ||emp_cur.salary);
END LOOP;
END;
/

Stored Procedures:

What is a Stored Procedure?

A stored procedure or in simple a proc is a named PL/SQL block which performs one or more specific task. This is similar to a procedure in other programming languages.

A procedure has a header and a body. The header consists of the name of the procedure and the parameters or variables passed to the procedure. The body consists or declaration section, execution section and exception section similar to a general PL/SQL Block.

A procedure is similar to an anonymous PL/SQL Block but it is named for repeated usage.

Procedures: Passing Parameters

We can pass parameters to procedures in three ways.
1) IN-parameters
2) OUT-parameters
3) IN OUT-parameters

A procedure may or may not return any value.

General Syntax to create a procedure is:
CREATE [OR REPLACE] PROCEDURE proc_name [list of parameters]
IS
   Declaration section
BEGIN
   Execution section
EXCEPTION
  Exception section
END;
Procedures: Example
CREATE OR REPLACE PROCEDURE employer_details
IS
CURSOR emp_cur IS
SELECT first_name, last_name, salary FROM emp_tbl;
emp_rec emp_cur%rowtype;
BEGIN
FOR emp_rec in sales_cur
LOOP
dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name
|| ' ' ||emp_cur.salary);
END LOOP;
END;
/
How to execute a Stored Procedure?
EXECUTE [or EXEC] procedure_name;
procedure_name;

PL/SQL Functions

What is a Function in PL/SQL?

A function is a named PL/SQL Block which is similar to a procedure. The major difference between a procedure and a function is, a function must always return a value, but a procedure may or may not return a value.

General Syntax to create a function is

CREATE [OR REPLACE] FUNCTION function_name [parameters]

RETURN return_datatype;

IS

Declaration_section

BEGIN

Execution_section

Return return_variable;

EXCEPTION

exception section

Return return_variable;

END;

2) The execution and exception section both should return a value which is of the datatype defined in the header section.
CREATE OR REPLACE FUNCTION employer_details_func
RETURN VARCHAR(20);
IS
emp_name VARCHAR(20);
BEGIN
SELECT first_name INTO emp_name FROM emp_tbl WHERE empID = '100';
RETURN emp_name;
END;
/
The return type of the function is VARCHAR which is declared in line no 2.
The function returns the 'emp_name' which is of type VARCHAR as the return value in line no 9.
How to execute a PL/SQL Function?
employee_name := employer_details_func;
SELECT employer_details_func FROM dual;
dbms_output.put_line(employer_details_func);

Parameters in Procedure and Functions

How to pass parameters to Procedures and Functions in PL/SQL?

In PL/SQL, we can pass parameters to procedures and functions in three ways.

1) IN type parameter: These types of parameters are used to send values to stored procedures.
2) OUT type parameter: These types of parameters are used to get values from stored procedures. This is similar to a return type in functions.
3) IN OUT parameter: These types of parameters are used to send values and get values from stored procedures.

NOTE: If a parameter is not explicitly defined a parameter type, then by default it is an IN type parameter.

1) IN parameter:

This is similar to passing parameters in programming languages. We can pass values to the stored procedure through these parameters or variables. This type of parameter is a read only parameter. We can assign the value of IN type parameter to a variable or use it in a query, but we cannot change its value inside the procedure.

General syntax to pass a IN parameter is

param_name1, param_name2... are unique parameter names.

datatype - defines the datatype of the variable.

IN - is optional, by default it is a IN type parameter.

2) OUT Parameter:
3) IN OUT Parameter:
In line no 3; we are creating a cursor ‘id_cur’ which contains the employee id.
In line no 7; we are calling the procedure ‘emp_name’, we are passing the ‘id’ as IN parameter and ‘empName’ as OUT parameter.
In line no 8; we are displaying the id and the employee name which we got from the procedure ‘emp_name’.

Exception Handling

In this section we will discuss about the following,
1) What is Exception Handling.
2) Structure of Exception Handling.
3) Types of Exception Handling.

1) What is Exception Handling?

PL/SQL provides a feature to handle the Exceptions which occur in a PL/SQL Block known as exception Handling. Using Exception Handling we can test the code and avoid it from exiting abruptly.

When an exception occurs a messages which explains its cause is recieved.
PL/SQL Exception message consists of three parts.
1) Type of Exception
2) An Error Code
3) A message

By Handling the exceptions we can ensure a PL/SQL block does not exit abruptly.

2) Structure of Exception Handling.
General Syntax for coding the exception section
DECLARE
   Declaration section
BEGIN
   Exception section
EXCEPTION
WHEN ex_name1 THEN
    -Error handling statements
WHEN ex_name2 THEN
    -Error handling statements
WHEN Others THEN
   -Error handling statements
END;
General PL/SQL statments can be used in the Exception Block.
DELCARE
   Declaration section
BEGIN
    DECLARE
      Declaration section
    BEGIN
      Execution section
    EXCEPTION
      Exception section
    END;
EXCEPTION
   Exception section
END;
3) Types of Exception.
a) Named System Exceptions
b) Unnamed System Exceptions
c) User-defined Exceptions
a) Named System Exceptions
1) Not Declared explicitly,
2) Raised implicitly when a predefined Oracle error occurs,
3) caught by referencing the standard name within an exception-handling routine.

Exception Name	Reason	Error Number
CURSOR_ALREADY_OPEN	When you open a cursor that is already open.	ORA-06511
INVALID_CURSOR	When you perform an invalid operation on a cursor like closing a cursor, fetch data from a cursor that is not opened.	ORA-01001
NO_DATA_FOUND	When a SELECT...INTO clause does not return any row from a table.	ORA-01403
TOO_MANY_ROWS	When you SELECT or fetch more than one row into a record or variable.	ORA-01422
ZERO_DIVIDE	When you attempt to divide a number by zero.	ORA-01476

BEGIN
  Execution section
EXCEPTION
WHEN NO_DATA_FOUND THEN
dbms_output.put_line ('A SELECT...INTO did not return any row.');
END;
b) Unnamed System Exceptions
1. By using the WHEN OTHERS exception handler, or
2. By associating the exception code to a name and using it as a named exception.
EXCEPTION_INIT will associate a predefined Oracle error number to a programmer_defined exception name.
• They are raised implicitly.
• If they are not handled in WHEN Others they must be handled explicity.
• To handle the exception explicity, they must be declared using Pragma EXCEPTION_INIT as given above and handled referecing the user-defined exception name in the exception section.
DECLARE
   exception_name EXCEPTION;
   PRAGMA
   EXCEPTION_INIT (exception_name, Err_code);
BEGIN
Execution section
EXCEPTION
WHEN exception_name THEN
     handle the exception
END;
If we try to delete a product_id from the product table when it has child records in order_id table an exception will be thrown with oracle code number -2292.
We can provide a name to this exception and handle it in the exception section as given below.
DECLARE
  Child_rec_exception EXCEPTION;
  PRAGMA
   EXCEPTION_INIT (Child_rec_exception, -2292);
BEGIN
  Delete FROM product where product_id= 104;
EXCEPTION
   WHEN Child_rec_exception
   THEN Dbms_output.put_line('Child records are present for this product_id.');
END;
/
c) User-defined Exceptions
• They should be explicitly declared in the declaration section.
• They should be explicitly raised in the Execution Section.
• They should be handled by referencing the user-defined exception name in the exception section.
Lets create a business rule that if the total no of units of any particular product sold is more than 20, then it is a huge quantity and a special discount should be provided.
DECLARE
  huge_quantity EXCEPTION;
  CURSOR product_quantity is
  SELECT p.product_name as name, sum(o.total_units) as units
FROM order_tems o, product p
WHERE o.product_id = p.product_id;
  quantity order_tems.total_units%type;
  up_limit CONSTANT order_tems.total_units%type := 20;
  message VARCHAR2(50);
BEGIN
  FOR product_rec in product_quantity LOOP
    quantity := product_rec.units;
     IF quantity > up_limit THEN
      message := 'The number of units of product ' || product_rec.name ||
                 ' is more than 20. Special discounts should be provided.
              Rest of the records are skipped. '
     RAISE huge_quantity;
     ELSIF quantity < up_limit THEN
      v_message:= 'The number of unit is below the discount limit.';
     END IF;
     dbms_output.put_line (message);
  END LOOP;
EXCEPTION
   WHEN huge_quantity THEN
     dbms_output.put_line (message);
END;
/
RAISE_APPLICATION_ERROR ( )
a) to create a unique id for an user-defined exception.
b) to make the user-defined exception look like an Oracle error.
RAISE_APPLICATION_ERROR (error_number, error_message);
• The Error number must be between -20000 and -20999
• The Error_message is the message you want to display when the error occurs.
1. Declare a user-defined exception in the declaration section.
2. Raise the user-defined exception based on a specific business rule in the execution section.
3. Finally, catch the exception and link the exception to a user-defined error number in RAISE_APPLICATION_ERROR.
DECLARE
huge_quantity EXCEPTION;
  CURSOR product_quantity is
  SELECT p.product_name as name, sum(o.total_units) as units
FROM order_tems o, product p
WHERE o.product_id = p.product_id;
  quantity order_tems.total_units%type;
  up_limit CONSTANT order_tems.total_units%type := 20;
  message VARCHAR2(50);
BEGIN
  FOR product_rec in product_quantity LOOP
    quantity := product_rec.units;
     IF quantity > up_limit THEN
        RAISE huge_quantity;
     ELSIF quantity < up_limit THEN
      v_message:= 'The number of unit is below the discount limit.';
     END IF;
     Dbms_output.put_line (message);
  END LOOP;
EXCEPTION
   WHEN huge_quantity THEN
       raise_application_error(-2100, 'The number of unit is above the discount limit.');
END;
/

A trigger is a pl/sql block structure which is fired when a DML statements like Insert, Delete, Update is executed on a database table. A trigger is triggered automatically when an associated DML statement is executed.

Syntax of Triggers

Syntax for Creating a Trigger

CREATE [OR REPLACE ] TRIGGER trigger_name

{BEFORE | AFTER | INSTEAD OF }

{INSERT [OR] | UPDATE [OR] | DELETE}

[OF col_name]

ON table_name

[REFERENCING OLD AS o NEW AS n]

[FOR EACH ROW]

WHEN (condition)

BEGIN

--- sql statements

END;

CREATE [OR REPLACE ] TRIGGER trigger_name - This clause creates a trigger with the given name or overwrites an existing trigger with the same name.

{BEFORE | AFTER | INSTEAD OF } - This clause indicates at what time should the trigger get fired. i.e for example: before or after updating a table. INSTEAD OF is used to create a trigger on a view. before and after cannot be used to create a trigger on a view.

{INSERT [OR] | UPDATE [OR] | DELETE} - This clause determines the triggering event. More than one triggering events can be used together separated by OR keyword. The trigger gets fired at all the specified triggering event.

[OF col_name] - This clause is used with update triggers. This clause is used when you want to trigger an event only when a specific column is updated.

CREATE [OR REPLACE ] TRIGGER trigger_name - This clause creates a trigger with the given name or overwrites an existing trigger with the same name.

[ON table_name] - This clause identifies the name of the table or view to which the trigger is associated.

[REFERENCING OLD AS o NEW AS n] - This clause is used to reference the old and new values of the data being changed. By default, you reference the values as :old.column_name or :new.column_name. The reference names can also be changed from old (or new) to any other user-defined name. You cannot reference old values when inserting a record, or new values when deleting a record, because they do not exist.

[FOR EACH ROW] - This clause is used to determine whether a trigger must fire when each row gets affected ( i.e. a Row Level Trigger) or just once when the entire sql statement is executed(i.e.statement level Trigger).

WHEN (condition) - This clause is valid only for row level triggers. The trigger is fired only for rows that satisfy the condition specified.

1) Row level trigger - An event is triggered for each row upated, inserted or deleted.
2) Statement level trigger - An event is triggered for each sql statement executed.
1) BEFORE statement trigger fires first.
2) Next BEFORE row level trigger fires, once for each row affected.
3) Then AFTER row level trigger fires once for each affected row. This events will alternates between BEFORE and AFTER row level triggers.
4) Finally the AFTER statement level trigger fires.
Before update, row level                    26-Nov-2008
After update, Row level                     26-Nov-2008
Before update, row level                    26-Nov-2008
After update, Row level                     26-Nov-2008
After update, statement level            26-Nov-2008
TRIGGER_TYPE                  VARCHAR2(16)
TRIGGER_EVENT                VARCHAR2(75)
TABLE_OWNER                 VARCHAR2(30)
BASE_OBJECT_TYPE           VARCHAR2(16)
TABLE_NAME                     VARCHAR2(30)
COLUMN_NAME                 VARCHAR2(4000)
REFERENCING_NAMES        VARCHAR2(128)
WHEN_CLAUSE                 VARCHAR2(4000)
STATUS                            VARCHAR2(8)
DESCRIPTION                    VARCHAR2(4000)
ACTION_TYPE                   VARCHAR2(11)
TRIGGER_BODY                 LONG
Let's consider we have two tables 'abc' and 'xyz'. Two triggers are created.
1) The INSERT Trigger, triggerA on table 'abc' issues an UPDATE on table 'xyz'.
2) The UPDATE Trigger, triggerB on table 'xyz' issues an INSERT on table 'abc'.
When the table 'xyz' is updated, triggerB fires and will insert a row in table 'abc'.
This cyclic situation continues and will enter into a infinite loop, which will crash the database.

A Simple PL/SQL Block:

Each PL/SQL program consists of SQL and PL/SQL statements which from a PL/SQL block.

PL/SQL Block consists of three sections:

Declaration Section:

Execution Section:

Exception Section:

How a Sample PL/SQL Block Looks

SQL Command Categories

SQL commands are grouped into four major categories depending on their functionality. They are as follows:

Data Definition Language (DDL)

These SQL commands are used for creating, modifying, and dropping the structure of database objects. The commands are CREATE, ALTER, DROP, RENAME, and TRUNCATE.

Data Manipulation Language (DML)

These SQL commands are used for storing, retrieving, modifying, and deleting data. These commands are SELECT, INSERT, UPDATE, and DELETE.

Transaction Control Language (TCL)

These SQL commands are used for managing changes affecting the data. These commands are COMMIT, ROLLBACK, and SAVEPOINT.

Data Control Language (DCL)

These SQL commands are used for providing security to database objects. These commands are GRANT and REVOKE.

Advantages of PL/SQL

For example: The below example declares two variables, one of which is a not null.

DECLARE

salary number(4);

dept varchar2(10) NOT NULL := “HR Dept”;

The value of a variable can change in the execution or exception section of the PL/SQL Block. We can assign values to variables in the two ways given below.

1) We can directly assign values to variables.

variable_name:= value;

2) We can assign values to variables directly from the database columns by using a SELECT.. INTO statement. The General Syntax is:

Example: The below program will get the salary of an employee with id '1116' and display it on the screen.

DECLARE

var_salary number(6);

var_emp_id number(6) = 1116;

BEGIN

SELECT salary

INTO var_salary

FROM employee

WHERE emp_id = var_emp_id;

dbms_output.put_line(var_salary);

dbms_output.put_line('The employee '

|| var_emp_id || ' has salary ' || var_salary);

END;

NOTE: The backward slash '/' in the above program indicates to execute the above PL/SQL Block.

Scope of PS/SQL Variables

PL/SQL allows the nesting of Blocks within Blocks i.e, the Execution section of an outer block can contain inner blocks. Therefore, a variable which is accessible to an outer Block is also accessible to all nested inner Blocks. The variables declared in the inner blocks are not accessible to outer blocks. Based on their declaration we can classify variables into two types.

For Example: In the below example we are creating two variables in the outer block and assigning thier product to the third variable created in the inner block. The variable 'var_mult' is declared in the inner block, so cannot be accessed in the outer block i.e. it cannot be accessed after line 11. The variables 'var_num1' and 'var_num2' can be accessed anywhere in the block.

PL/SQL Constants

PL/SQL Records

By declaring the field datatype in the above method, the datatype of the column is dynamically applied to the field. This method is useful when you are altering the column specification of the table, because you do not need to change the code again.

NOTE: You can use also %type to declare variables and constants.

The following code shows how to declare a record called employee_rec based on a user-defined type.

If all the fields of a record are based on the columns of a table, we can declare the record as follows:

For example, the above declaration of employee_rec can as follows:

The advantages of declaring the record as a ROWTYPE are:

The disadvantage of declaring the record as a ROWTYPE is:

NOTE: When you are creating a record, you are just creating a datatype, similar to creating a variable. You need to assign values to the record to use them.

Passing Values To and From a Record

When you assign values to a record, you actually assign values to the fields within it.

If you used %ROWTYPE to declare a record, you can assign values as shown:

We can assign values to records using SELECT Statements as shown:

If %ROWTYPE is used to declare a record then you can directly assign values to the whole record instead of each columns separately. In this case, you must SELECT all the columns from the table into the record as shown:

Lets see how we can get values from a record.

The following table consolidates the different ways you can assign values to and from a record:

Iterative Statements in PL/SQL

General Syntax to write a Simple Loop is:

LOOP

statements;

EXIT;

{or EXIT WHEN condition;}

END LOOP;

These are the important steps to be followed while using Simple Loop.

1) Initialise a variable before the loop body.

2) While Loop

The General Syntax to write a WHILE LOOP is:

WHILE <condition>

LOOP statements;

END LOOP;

Important steps to follow when executing a while loop:

1) Initialise a variable before the loop body.

3) FOR Loop

The General Syntax to write a FOR LOOP is:

FOR counter IN val1..val2

LOOP statements;

END LOOP;

Important steps to follow when executing a while loop:

1) The counter variable is implicitly declared in the declaration section, so it's not necessary to declare it explicity.

NOTE: The above Loops are explained with a example when dealing with Explicit Cursors.

When you execute DML statements like DELETE, INSERT, UPDATE and SELECT statements, implicit statements are created to process these statements.

Oracle provides few attributes called as implicit cursor attributes to check the status of DML operations. The cursor attributes available are %FOUND, %NOTFOUND, %ROWCOUNT, and %ISOPEN.

For example, When you execute INSERT, UPDATE, or DELETE statements the cursor attributes tell us whether any rows are affected and how many have been affected.

The status of the cursor for each of these attributes are defined in the below table.

For Example: Consider the PL/SQL Block that uses implicit cursor attributes as shown below:

In the above PL/SQL Block, the salaries of all the employees in the ‘employee’ table are updated. If none of the employee’s salary are updated we get a message 'None of the salaries where updated'. Else we get a message like for example, 'Salaries for 1000 employees are updated' if there are 1000 rows in ‘employee’ table.

1) Declaring a Cursor in the Declaration Section:

In the above example we are creating a cursor ‘emp_cur’ on a query which returns the records of all the

2) Accessing the records in the cursor:

These are the three steps in accessing the cursor.

General Syntax to open a cursor is:

General Syntax to fetch records from a cursor is:

General Syntax to close a cursor is:

When a cursor is opened, the first row becomes the current row. When the data is fetched it is copied to the record or variables and the logical pointer moves to the next row and it becomes the current row. On every fetch statement, the pointer moves to the next row. If you want to fetch after the last row, the program will throw an error. When there is more than one row in a cursor we can use loops along with explicit cursor attributes to fetch all the records.

Points to remember while fetching a row:

· We can fetch the rows in a cursor to a PL/SQL Record or a list of variables created in the PL/SQL Block.

General Form of using an explicit cursor is:

In the above example, first we are creating a record ‘emp_rec’ of the same structure as of table ‘emp_tbl’ in line no 2. We can also create a record with a cursor by replacing the table name with the cursor name. Second, we are declaring a cursor ‘emp_cur’ from a select query in line no 3 - 6. Third, we are opening the cursor in the execution section in line no 8. Fourth, we are fetching the cursor to the record in line no 9. Fifth, we are displaying the first_name and last_name of the employee in the record emp_rec in line no 10. Sixth, we are closing the cursor in line no 11.

Oracle provides some attributes known as Explicit Cursor Attributes to control the data processing while using cursors. We use these attributes to avoid errors while accessing cursors through OPEN, FETCH and CLOSE Statements.

a) When we try to open a cursor which is not closed in the previous operation.

These are the attributes available to check the status of an explicit cursor.

Oracle provides three types of cursors namely SIMPLE LOOP, WHILE LOOP and FOR LOOP. These loops can be used to process multiple rows in the cursor. Here I will modify the same example for each loops to explain how to use loops with cursors.

In the above example we are using two cursor attributes %ISOPEN and %NOTFOUND.

We can use %FOUND in place of %NOTFOUND and vice versa. If we do so, we need to reverse the logic of the program. So use these attributes in appropriate instances.

Lets modify the above program to use while loop.

In the above example, in line no 10 we are using %FOUND to evaluate if the first fetch statement in line no 9 returned a row, if true the program moves into the while loop. In the loop we use fetch statement again (line no 15) to process the next row. If the fetch statement is not executed once before the while loop the while condition will return false in the first instance and the while loop is skipped. In the loop, before fetching the record again, always process the record retrieved by the first fetch statement, else you will skip the first row.

When using FOR LOOP you need not declare a record or variables to store the cursor values, need not open, fetch and close the cursor. These functions are accomplished by the FOR LOOP automatically.

General Syntax for using FOR LOOP:

Let’s use the above example to learn how to use for loops in cursors.

In the above example, when the FOR loop is processed a record ‘emp_rec’of structure ‘emp_cur’ gets created, the cursor is opened, the rows are fetched to the record ‘emp_rec’ and the cursor is closed after the last row is processed. By using FOR Loop in your program, you can reduce the number of lines in the program.

NOTE: In the examples given above, we are using backward slash ‘/’ at the end of the program. This indicates the oracle engine that the PL/SQL program has ended and it can begin processing the statements.

IS - marks the beginning of the body of the procedure and is similar to DECLARE in anonymous PL/SQL Blocks. The code between IS and BEGIN forms the Declaration section.

The syntax within the brackets [ ] indicate they are optional. By using CREATE OR REPLACE together the procedure is created if no other procedure with the same name exists or the existing procedure is replaced with the current code.

The below example creates a procedure ‘employer_details’ which gives the details of the employee.

There are two ways to execute a procedure.

1) From the SQL prompt.

2) Within another procedure – simply use the procedure name.

1) Return Type: The header section defines the return type of the function. The return datatype can be any of the oracle datatype like varchar, number etc.

For example, let’s create a frunction called ''employer_details_func' similar to the one created in stored proc

In the example we are retrieving the ‘first_name’ of employee with empID 100 to variable ‘emp_name’.

A function can be executed in the following ways.

1) Since a function returns a value we can assign it to a variable.

If ‘employee_name’ is of datatype varchar we can store the name of the employee by assigning the return type of the function to it.

2) As a part of a SELECT statement

3) In a PL/SQL Statements like,

This line displays the value returned by the function.

CREATE [OR REPLACE] PROCEDURE procedure_name (

param_name1 IN datatype, param_name12 IN datatype ... )

The OUT parameters are used to send the OUTPUT from a procedure or a function. This is a write-only parameter i.e, we cannot pass values to OUT paramters while executing the stored procedure, but we can assign values to OUT parameter inside the stored procedure and the calling program can recieve this output value.

The General syntax to create an OUT parameter is

CREATE [OR REPLACE] PROCEDURE proc2 (param_name OUT datatype)

The parameter should be explicity declared as OUT parameter.

The IN OUT parameter allows us to pass values into a procedure and get output values from the procedure. This parameter is used if the value of the IN parameter can be changed in the calling program.

By using IN OUT parameter we can pass values into a parameter and return a value to the calling program using the same parameter. But this is possible only if the value passed to the procedure and output value have a same datatype. This parameter is used if the value of the parameter will be changed in the procedure.

The General syntax to create an IN OUT parameter is

CREATE [OR REPLACE] PROCEDURE proc3 (param_name IN OUT datatype)

The below examples show how to create stored procedures using the above three types of parameters.

Example1:

Using IN and OUT parameter:

Let’s create a procedure which gets the name of the employee when the employee id is passed.

CREATE OR REPLACE PROCEDURE emp_name (id IN NUMBER, emp_name OUT NUMBER)

BEGIN

SELECT first_name INTO emp_name

FROM emp_tbl WHERE empID = id;

END;

We can call the procedure ‘emp_name’ in this way from a PL/SQL Block.

DECLARE

empName varchar(20);

CURSOR id_cur SELECT id FROM emp_ids;

BEGIN

FOR emp_rec in id_cur

LOOP

emp_name(emp_rec.id, empName);

dbms_output.putline('The employee ' || empName || ' has id ' || emp-rec.id);

END LOOP;

END;

In the above PL/SQL Block

Example 2:

Using IN OUT parameter in procedures:

CREATE OR REPLACE PROCEDURE emp_salary_increase

(emp_id IN emptbl.empID%type, salary_inc IN OUT emptbl.salary%type)

tmp_sal number;

BEGIN

SELECT salary INTO tmp_sal FROM emp_tbl WHERE empID = emp_id;

IF tmp_sal between 10000 and 20000 THEN

salary_inout := tmp_sal * 1.2;

ELSIF tmp_sal between 20000 and 30000 THEN

salary_inout := tmp_sal * 1.3;

ELSIF tmp_sal > 30000 THEN

salary_inout := tmp_sal * 1.4;

END IF;

END;

The below PL/SQL block shows how to execute the above 'emp_salary_increase' procedure.

DECLARE

CURSOR updated_sal is

SELECT empID,salary FROM emp_tbl;

pre_sal number;

BEGIN

FOR emp_rec IN updated_sal LOOP

pre_sal := emp_rec.salary;

emp_salary_increase(emp_rec.empID, emp_rec.salary);

dbms_output.put_line('The salary of ' || emp_rec.empID ||

' increased from '|| pre_sal || ' to '||emp_rec.salary);

END LOOP;

END;

When an exception is raised, Oracle searches for an appropriate exception handler in the exception section. For example in the above example, if the error raised is 'ex_name1 ', then the error is handled according to the statements under it. Since, it is not possible to determine all the possible runtime errors during testing fo the code, the 'WHEN Others' exception is used to manage the exceptions that are not explicitly handled. Only one exception can be raised in a Block and the control does not return to the Execution Section after the error is handled.

If there are nested PL/SQL blocks like this.

In the above case, if the exception is raised in the inner block it should be handled in the exception block of the inner PL/SQL block else the control moves to the Exception block of the next upper PL/SQL Block. If none of the blocks handle the exception the program ends abruptly with an error.

There are 3 types of Exceptions.

System exceptions are automatically raised by Oracle, when a program violates a RDBMS rule. There are some system exceptions which are raised frequently, so they are pre-defined and given a name in Oracle which are known as Named System Exceptions.

For example: NO_DATA_FOUND and ZERO_DIVIDE are called Named System exceptions.

Named system exceptions are:

For Example: Suppose a NO_DATA_FOUND exception is raised in a proc, we can write a code to handle the exception as given below.

Those system exception for which oracle does not provide a name is known as unamed system exception. These exception do not occur frequently. These Exceptions have a code and an associated message.

There are two ways to handle unnamed sysyem exceptions:

We can assign a name to unnamed system exceptions using a Pragma called EXCEPTION_INIT.

Steps to be followed to use unnamed system exceptions are

The general syntax to declare unnamed system exception using EXCEPTION_INIT is:

For Example: Lets consider the product table and order_items table from sql joins.

Here product_id is a primary key in product table and a foreign key in order_items table.

Apart from sytem exceptions we can explicity define exceptions based on business rules. These are known as user-defined exceptions.

Steps to be followed to use user-defined exceptions:

For Example: Lets consider the product table and order_items table from sql joins to explain user-defined exception.

RAISE_APPLICATION_ERROR is a built-in procedure in oracle which is used to display the user-defined error messages along with the error number whose range is in between -20000 and -20999.

Whenever a message is displayed using RAISE_APPLICATION_ERROR, all previous transactions which are not committed within the PL/SQL Block are rolled back automatically (i.e. change due to INSERT, UPDATE, or DELETE statements).

RAISE_APPLICATION_ERROR raises an exception but does not handle it.

RAISE_APPLICATION_ERROR is used for the following reasons,

The General Syntax to use this procedure is:

Steps to be folowed to use RAISE_APPLICATION_ERROR procedure:

Using the above example we can display a error message using RAISE_APPLICATION_ERROR.

What is a Trigger?

For Example: The price of a product changes constantly. It is important to maintain the history of the prices of the products.

We can create a trigger to update the 'product_price_history' table when the price of the product is updated in the 'product' table.

1) Create the 'product' table and 'product_price_history' table

CREATE TABLE product_price_history

(product_id number(5),

product_name varchar2(32),

supplier_name varchar2(32),

unit_price number(7,2) );

CREATE TABLE product

(product_id number(5),

product_name varchar2(32),

supplier_name varchar2(32),

unit_price number(7,2) );

2) Create the price_history_trigger and execute it.

CREATE or REPLACE TRIGGER price_history_trigger

BEFORE UPDATE OF unit_price

ON product

FOR EACH ROW

BEGIN

INSERT INTO product_price_history

VALUES

(:old.product_id,

:old.product_name,

:old.supplier_name,

:old.unit_price);

END;

3) Lets update the price of a product.

UPDATE PRODUCT SET unit_price = 800 WHERE product_id = 100

Once the above update query is executed, the trigger fires and updates the 'product_price_history' table.

4)If you ROLLBACK the transaction before committing to the database, the data inserted to the table is also rolled back.

Types of PL/SQL Triggers

There are two types of triggers based on the which level it is triggered.

PL/SQL Trigger Execution Hierarchy

The following hierarchy is followed when a trigger is fired.

For Example: Let's create a table 'product_check' which we can use to store messages when triggers are fired.

CREATE TABLE product

(Message varchar2(50),

Current_Date number(32)

);

Let's create a BEFORE and AFTER statement and row level triggers for the product table.

1) BEFORE UPDATE, Statement Level: This trigger will insert a record into the table 'product_check' before a sql update statement is executed, at the statement level.

CREATE or REPLACE TRIGGER Before_Update_Stat_product

BEFORE

UPDATE ON product

Begin

INSERT INTO product_check

Values('Before update, statement level',sysdate);

END;

2) BEFORE UPDATE, Row Level: This trigger will insert a record into the table 'product_check' before each row is updated.

CREATE or REPLACE TRIGGER Before_Upddate_Row_product

BEFORE

UPDATE ON product

FOR EACH ROW

BEGIN

INSERT INTO product_check

Values('Before update row level',sysdate);

END;

3) AFTER UPDATE, Statement Level: This trigger will insert a record into the table 'product_check' after a sql update statement is executed, at the statement level.

CREATE or REPLACE TRIGGER After_Update_Stat_product

AFTER

UPDATE ON product

BEGIN

INSERT INTO product_check

Values('After update, statement level', sysdate);

End;

4) AFTER UPDATE, Row Level: This trigger will insert a record into the table 'product_check' after each row is updated.

CREATE or REPLACE TRIGGER After_Update_Row_product

AFTER

insert On product

FOR EACH ROW

BEGIN

INSERT INTO product_check

Values('After update, Row level',sysdate);

END;

Now lets execute a update statement on table product.

UPDATE PRODUCT SET unit_price = 800

WHERE product_id in (100,101);

Lets check the data in 'product_check' table to see the order in which the trigger is fired.

SELECT * FROM product_check;

Output:

Mesage Current_Date

------------------------------------------------------------

Before update, statement level 26-Nov-2008

The above result shows 'before update' and 'after update' row level events have occured twice, since two records were updated. But 'before update' and 'after update' statement level events are fired only once per sql statement.

The above rules apply similarly for INSERT and DELETE statements.

How To know Information about Triggers.

We can use the data dictionary view 'USER_TRIGGERS' to obtain information about any trigger.

The below statement shows the structure of the view 'USER_TRIGGERS'

DESC USER_TRIGGERS;

NAME Type

--------------------------------------------------------

TRIGGER_NAME VARCHAR2(30)

This view stores information about header and body of the trigger.

SELECT * FROM user_triggers WHERE trigger_name = 'Before_Update_Stat_product';

The above sql query provides the header and body of the trigger 'Before_Update_Stat_product'.

You can drop a trigger using the following command.

DROP TRIGGER trigger_name;

CYCLIC CASCADING in a TRIGGER

This is an undesirable situation where more than one trigger enter into an infinite loop. while creating a trigger we should ensure the such a situtation does not exist.

The below example shows how Trigger's can enter into cyclic cascading.

In such a situation, when there is a row inserted in table 'abc', triggerA fires and will update table 'xyz'.

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PL/SQL Basic

Conditional Statements in PL/SQL

Conditional Statements in PL/SQL

What are Cursors?

Implicit cursors:

Explicit cursors:

Explicit Cursors

How to use Explicit Cursor?

There are four steps in using an Explicit Cursor.

Stored Procedures:

What is a Stored Procedure?

Procedures: Passing Parameters

PL/SQL Functions

What is a Function in PL/SQL?

General Syntax to create a function is

Exception Handling

1) What is Exception Handling?