What is a trigger?
#Triggers are used to "automatically" perform some operations after/before certain operations. (For example, if new student information is inserted, the number of students should be modified in the class table).
After insert delete update sets the trigger, executing the insert delete update operation will automatically trigger the set content.
A table can have up to 6 triggers (3*2, three operations*two times (before|after)).
Create trigger
Syntax:
create trigger 触发器名 before|after 事件 on 表名 for each row 触发器语句;
The trigger name is recommended to be trigger_xxx, so that it is easy to distinguish,trigger names cannot be repeated.
before|after represents the trigger statement execution time. If it is before, the trigger statement is executed before the insert delete update operation; after is after.
The event is one of the insert delete update operations.
for each row is a trigger that will trigger the corresponding operation on behalf of any record.
The trigger statement is the statement to be executed when the trigger is fired.
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