TRIGGERS in PL/SQL.

PL/SQL - TRIGGERS

Triggers are stored programs, which are automatically executed or fired when some events occur.
Triggers are, in fact, written to be executed in response to any of the following events:
▪︎A database manipulation DML statement DELETE, INSERT, orUPDATE.
▪︎ A database definition DDL statement CREATE, ALTER, orDROP.
▪︎ A database operation SERVERERROR, LOGON, LOGOFF, STARTUP, orSHUTDOWN.

Triggers could be defined on the table, view, schema, or database with which the event is associated.

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Benefits of Triggers:

Triggers can be written for the following purposes:
• Generating some derived column values automatically.
• Enforcing referential integrity.
• Event logging and storing information on table access.
• Auditing.
• Synchronous replication of tables.
• Imposing security authorizations.
• Preventing invalid transactions.

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Creating Triggers:

The syntax for creating a trigger is:

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) DECLARE Declaration-statements BEGIN Executable-statements EXCEPTION Exception-handling-statements END;

Where,
□ CREATE [OR REPLACE] TRIGGER trigger_name: Creates or replaces an existing trigger with the trigger_name.
□ {BEFORE | AFTER | INSTEAD OF} : This specifies when the trigger would be executed.
□ The INSTEAD OF clause is used for creating trigger on a view.
□ {INSERT [OR] | UPDATE [OR] | DELETE}: This specifies the DML operation.
□ [OF col_name]: This specifies the column name that would be updated.
□[ON table_name]: This specifies the name of the table associated with the trigger.
□ [REFERENCING OLD AS o NEW AS n]: This allows you to refer new and old values for various DML statements, like INSERT, UPDATE, and DELETE.
□ [FOR EACH ROW]: This specifies a row level trigger, i.e., the trigger would be executed for each row being affected. Otherwise the trigger will execute just once when the SQL statement is executed, which is called a table level trigger.
□ WHEN condition: This provides a condition for rows for which the trigger would fire. This clause is valid only for row level triggers.

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