Triggers

A trigger is a statement that is executed automatically by the system as a side effect of a modification to the database.

Specify when a trigger is to be executed. This is broken up into an event that causes the trigger to be checked and a condition that must be satisfied for trigger execution to proceed.
Specify the actions to be taken when the trigger executes.
触发的时间和事件
触发时的动作
Triggering event can be insert, delete or update.
Triggers can be activated before/after an event

create trigger setnull-trigger before/after update on r
	referencing new row as nrow
	for each row
		when nrow.phone-number = ''
		set nrow.phone-numbwer = null;

空值需要明确地赋予

Instead of executing a separate action for each affected row, a single action can be executed for all rows affected by a single transaction

Use for each statement(语句触发器) instead of for each row(行触发器)
Use referencing old table or referencing new table to refer to temporary tables containing the afected rows
Can be more efficient when dealing with SQL statements that update a large number of rows

行触发器：如果modification涉及到n个行，则会触发n次语句触发器：不管涉及多少行，对update只执行n次，则会触发n次

Values of attributes before and after an update can be referenced

referencing old row as:for deletes and updates
referencing new row as:for inserts and updates

E.g.

create trigger credits_earned after update of takes on (grade)
referencing new row as nrow
referencing old row as orow
for each row
when nrow.grade <> 'F' and nrow.grade is no null
	and (orow.grade = 'F' or orow.grade is null)
begin atomic
	update student
	set tot_cred = tot_cred +
		(select credits
		 from course
		 where course.course_id = nrow.course_id)
	where student.id = nrow.id;
end;