Notes -8: mysql trigger

1. Introduction Triggers

A trigger is a database object is assigned to a linked table, when a specific event occurs on a table, it will be activated.
mysqk trigger only supports: INSERT, UPDATE, DELETE statements.
Each table each event allows only a trigger, so each table supports up to six triggers.

2. Create Trigger

the Create  
            TRIGGER trigger_name trigger_time trigger_event
             ON tb_name the FOR EACH ROW trigger_body 

# trigger_name: trigger name. 
# Trigger_time: when the trigger is triggered. That the use of DEFORE, AFTER trigger is used to indicate a trigger before or after the statement that activated it. If you want to verify that the new data to meet the restrictions on the use, use BEFORE; or if you want to complete a few more changes after activation of the trigger statement is executed by using AFTER. 
# Trigger_event: the triggering event for the kind of statement specifies that activates the trigger. ( INSERT , UPDATE , DELETE ). 
# Tb_name: table name associated with the trigger. 
# The FOR EACH ROW: designated for each row affected by the triggering event trigger actions to be activated. 
# Trigger_body: trigger action body contains mysql statement trigger when activated will be executed.

# Create a trigger tb_student_insert_trigger in the table tb_student, for every time you insert a row of data into a table tb_student in the value of the variable str students to " ON Student added!"
 The Create  the Trigger tb_student_insert_trigger the After INSERT  ON tb_student the FOR EACH ROW the SET @ str = ' ON Student added! ' ; 

INSERT  INTO tb_student values ( ' 2013110101 ' , ' Zhang Xiaoyong ' , ' M ' , ' 1997-12-11 ' , ' Shanxi ' ,' Chinese ' , ' AC1301 ' ); 

SELECT @ STR ; # verification trigger

3. Check the trigger

SHOW TRIGGERS [{FROM | IN} db_name]

4. Delete Trigger

DROP TRIGGER [IF EXISTS] trigger_name

Delete tb_student trigger tb_student_insert_trigger table
 drop  the Trigger  IF  EXISTS tb_student_trigger;

When you delete a table, the table will automatically delete trigger associated.
Triggers can not be updated with the cover, if you need to modify the trigger, you must first remove and then re-created.

5. Trigger

5.1 INSERT trigger

insert triggers may be performed before or after the insert statement execution. Need some attention:

You can refer to a virtual table at the NEW code to access the trigger insert rows are inserted.
In the before insert trigger, NEW values can also be updated, i.e., allowed to change the value to be inserted.
In AUTO_INCREMENT column, NEW insert before execution is 0, the execution after the insert.

# In the table tb_student recreate the trigger tb_student_insert_trigger, for every time you insert a row into a table tb_student in the value of the variable str students to learn new insert number of students.
Create  Trigger tb_student_insert_trigger an AFTER the INSERT  ON tb_student the FOR EACH the ROW the SET @ STR = NEW.studentNo;

5.2 DELETE trigger

DELETE DELETE triggers can be performed before or after the statement is executed. Need some attention:

In the DELETE trigger code can reference a virtual table named OLD to access the deleted rows.
OLD value in all is read, it can not be updated.

5.3 UPDATE trigger

UPDATE UPDATE statement trigger is executed before or after execution. Need some attention:

In the UPDATE trigger code can reference a table named OLD virtual access to the previous value, can also refer to a virtual table called NEW accessing the value of new updates.
In the BEFORE UPDATE trigger, the NEW value might also be updated, allowing to change the value of the UPDATE statement to be used.
OLD value in all is read-only, can not be updated.
When the trigger operation involves updating the table itself, use only BEFORE UPDATE trigger, AFTER UPDATE triggers can not be used.

# Create a table tb_student tb_student_update_trigger the trigger, the value for each update table a value column tb_student nation native column.
Create  Trigger tb_student_update_trigger the BEFORE the UPDATE  the ON tb_student the FOR EACH the ROW the SET NEW.nation = OLD.native; 

Update tb_student the SET Nation = ' strong '  WHERE studentName = ' Zhang Xiaoyong ' ;