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Triggers in SQL

Answer»

SQL codes automatically executed in response to a certain event occurring in a table of a database are called triggers. There cannot be more than 1 trigger with a similar action time and event for one table.

Syntax:

Create Trigger Trigger_Name
(Before | After) [ Insert | Update | Delete]
on [Table_Name]
[ for each row | for each column ]
[ trigger_body ]

Example:

CREATE TRIGGER trigger1
before INSERT
ON Student
FOR EACH ROW
SET new.total = (new.marks/ 10) * 100;

Here, we create a new Trigger called trigger1, just before we perform an INSERT operation on the Student table, we calculate the percentage of the marks for each row.
Some common operations that can be performed on triggers are:


  • DROP: This operation will drop an already existing trigger from the table.
    • Syntax:

DROP TRIGGER trigger name;

  • SHOW: This will display all the triggers that are currently present in the table.
    • Syntax:

SHOW TRIGGERS IN database_name;


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