Database composition trigger

17.1　 Create trigger

MySQL You can use CREATE TRIGGER sentence .MySQL Triggers in can contain an execution statement , It can also contain multiple execution statements .

17.1.1　 Grammar format

The syntax format for creating triggers is as follows ：

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CREATE      [DEFINER = user]      TRIGGER trigger_name      trigger_time trigger_event      ON tbl_name FOR EACH ROW      [trigger_order]      trigger_body  

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The syntax format is described as follows ：

·trigger_name： Name of the trigger created .

·trigger_time： Identify when to execute the trigger , Two options are supported , Respectively BEFORE and AFTER. among ,BEFORE Means to trigger before an event ,AFTER It means to trigger after an event .

·trigger_event： Triggered events , Support INSERT、UPDATE and DELETE operation .

·tbl_name： Data table name , Indicates on which data table the trigger is created .

·trigger_body： Executed in trigger SQL sentence , There can be one SQL sentence , It can also be multiple SQL sentence .

17.1.2　 Create trigger example

Triggers can be triggered before an event occurs , It can also be triggered after an event occurs .

1． Create a test data table

Create data table test_trigger and test_trigger_log.

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mysql> CREATE TABLE test_trigger (      -> id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,      -> t_note VARCHAR(30)      -> );  Query OK, 0 rows affected (0.12 sec)  mysql> CREATE TABLE test_trigger_log (      -> id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,      -> t_log VARCHAR(30)      -> );  Query OK, 0 rows affected (0.01 sec)  

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SQL Statement executed successfully , And then to test_trigger Add data table 、 Delete and modify operations , Use triggers to test_trigger The data change log in the data table is written test_trigger_log In the data table .

2．BEFORE INSERT trigger

for example , Create name as before_insert The trigger of , towards test_trigger Data table before inserting data , towards test_trigger_log Insert... Into the data table before_insert Log information .

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mysql> DELIMITER $$  mysql> CREATE TRIGGER before_insert      -> BEFORE INSERT ON test_trigger      -> FOR EACH ROW      -> INSERT INTO test_trigger_log       -> (t_log)      -> VALUES      -> ('before_insert')      -> ;      -> $$  Query OK, 0 rows affected (0.16 sec)  mysql> DELIMITER ;  

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SQL Statement executed successfully , towards test_trigger Insert data into the data table .

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mysql> INSERT INTO test_trigger (t_note) VALUES (' test  BEFORE INSERT  trigger ');  Query OK, 1 row affected (0.00 sec)  

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SQL Statement executed successfully , see test_trigger Data in the data table .

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mysql> SELECT * FROM test_trigger;  +----+--------------------------------+  | id | t_note                         |  +----+--------------------------------+  |  1 |  test  BEFORE INSERT  trigger       |  +----+--------------------------------+  1 row in set (0.01 sec)  

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towards test_trigger A piece of data was successfully inserted into the data table .

see test_trigger_log Data in the data table .

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mysql> SELECT * FROM test_trigger_log;  +----+---------------+  | id | t_log         |  +----+---------------+  |  1 | before_insert |  +----+---------------+  1 row in set (0.00 sec)  

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towards test_trigger Data table before inserting data , Execute trigger to test_trigger_log The data table is inserted before_insert Information .

3． establish AFTER INSERT trigger

for example , Create name as after_insert The trigger of , towards test_trigger After inserting data into the data table , towards test_trigger_log Insert... Into the data table after_insert Log information .

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mysql> DELIMITER $$  mysql> CREATE TRIGGER after_insert      -> AFTER INSERT ON test_trigger      -> FOR EACH ROW      -> BEGIN      -> INSERT INTO test_trigger_log       -> (t_log)      -> VALUES      -> ('after_insert');      -> END;      -> $$  Query OK, 0 rows affected (0.00 sec)  mysql> DELIMITER ;  

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towards test_trigger Insert data into the data table .

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mysql> INSERT INTO test_trigger (t_note) VALUES (' test  AFTER INSERT  trigger ');  Query OK, 1 row affected (0.00 sec)  

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SQL Statement executed successfully , see test_trigger Data in the data table .

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mysql> SELECT * FROM test_trigger;  +----+--------------------------------+  | id | t_note                         |  +----+--------------------------------+  |  1 |  test  BEFORE INSERT  trigger       |  |  2 |  test  AFTER INSERT  trigger        |  +----+--------------------------------+  2 rows in set (0.00 sec)  

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see test_trigger_log Data in the data table .

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mysql> SELECT * FROM test_trigger_log;  +----+---------------+  | id | t_log         |  +----+---------------+  |  1 | before_insert |  |  2 | before_insert |  |  3 | after_insert  |  +----+---------------+  3 rows in set (0.00 sec)  

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test_trigger_log A clause is inserted in the data table before_insert Information and a after_insert Information , This is because before_insert Trigger and after_insert trigger , Direction test_trigger When inserting data into a data table , Will execute first before_insert Trigger direction test_trigger_log Insert data into the data table , Re execution after_insert Trigger direction test_trigger_log Insert data into the data table , So they will ask test_trigger_log Insert two pieces of data into the data table .

Be careful ：BEFORE/AFTER UPDATE Trigger and BEFORE/AFTER DELETE The creation method of trigger is the same as BEFORE/AFTER INSERT Triggers are created in the same way , No more details here .

17.2　 Check triggers

MySQL Supported in SHOW TRIGGERS and SHOW CREATE TRIGGER Statement to view trigger information . meanwhile , stay MySQL Trigger information will be stored in information_schema In the database triggers In the data table , So it can also be in trigger View trigger information in the data table .

17.2.1　 Use SHOW TRIGGERS Statement to view trigger information

Use SHOW TRIGGERS Statement to view trigger information , The syntax is as follows ：

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SHOW TRIGGERS      [{FROM | IN} db_name]      [LIKE 'pattern' | WHERE expr]  

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·SHOW TRIGGERS： View trigger information SQL keyword .

·[{FROM | IN} db_name]：{FROM | IN} Indicates which database to view triggers from .db_name Represents the database name ; This item can be omitted , When omitted , View the current MySQL Trigger information of the database where the command line is located .

·[LIKE 'pattern' | WHERE expr]： Check the condition statement that matches when the trigger .

for example , View the current MySQL Trigger information under the database where the command line is located .

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mysql> SHOW TRIGGERS \G  *************************** 1. row ***************************               Trigger: before_insert                 Event: INSERT                 Table: test_trigger             Statement: INSERT INTO test_trigger_log  (t_log)  VALUES  ('before_insert')                Timing: BEFORE               Created: 2019-12-26 22:30:44.31              sql_mode: ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_  DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION               Definer: [email protected]  character_set_client: utf8mb4  collation_connection: utf8mb4_general_ci    Database Collation: utf8mb4_0900_ai_ci  *************************** 2. row ***************************               Trigger: after_insert                 Event: INSERT                 Table: test_trigger             Statement: BEGIN  INSERT INTO test_trigger_log   (t_log)  VALUES  ('after_insert');  END                Timing: AFTER               Created: 2019-12-26 22:41:57.51              sql_mode: ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_  DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION               Definer: [email protected]  character_set_client: utf8mb4  collation_connection: utf8mb4_general_ci    Database Collation: utf8mb4_0900_ai_ci    ################ Omit here n Line code ##########################    6 rows in set (0.00 sec)  

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The results show that MySQL All trigger information under the current database of the command line , And displays the name of each trigger 、 event 、 The data table where the trigger is located and triggered when the trigger is executed SQL Statement and other information .

17.2.2　 Use SHOW CREATE TRIGGER Statement to view trigger information

Use SHOW CREATE TRIGGER Statement to view trigger information , The syntax is as follows ：

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SHOW CREATE TRIGGER trigger_name  

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among ,trigger_name Indicates the name of the trigger .

for example , See the name before_insert Trigger information .

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mysql> SHOW CREATE TRIGGER before_insert \G  *************************** 1. row ***************************                 Trigger: before_insert                sql_mode: ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_  FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION  SQL Original Statement: CREATE DEFINER=`root`@`localhost` TRIGGER `before_insert` BEFORE INSERT ON   `test_trigger` FOR EACH ROW INSERT INTO test_trigger_log  (t_log)  VALUES  ('before_insert')    character_set_client: utf8mb4    collation_connection: utf8mb4_general_ci      Database Collation: utf8mb4_0900_ai_ci                 Created: 2019-12-26 22:30:44.31  1 row in set (0.44 sec)  

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The name of the trigger is displayed in the view result 、 Triggered when the trigger is executed SQL sentence .

17.2.3　 By looking at triggers Data in the data table view trigger information

stay MySQL in , The trigger information will be stored in information_schema In the database triggers In the data table . By looking at information_schema In the database triggers Use the data in the data table to view the trigger information .

The syntax is as follows ：

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SELECT * FROM information_schema.triggers  WHERE condition  

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for example , See the name before_insert Trigger information for .

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mysql> SELECT * FROM information_schema.triggers WHERE trigger_name = 'before_insert' \G  *************************** 1. row ***************************             TRIGGER_CATALOG: def              TRIGGER_SCHEMA: goods                TRIGGER_NAME: before_insert          EVENT_MANIPULATION: INSERT        EVENT_OBJECT_CATALOG: def         EVENT_OBJECT_SCHEMA: goods          EVENT_OBJECT_TABLE: test_trigger                ACTION_ORDER: 1            ACTION_CONDITION: NULL            ACTION_STATEMENT: INSERT INTO test_trigger_log  (t_log)  VALUES  ('before_insert')          ACTION_ORIENTATION: ROW               ACTION_TIMING: BEFORE  ACTION_REFERENCE_OLD_TABLE: NULL  ACTION_REFERENCE_NEW_TABLE: NULL    ACTION_REFERENCE_OLD_ROW: OLD    ACTION_REFERENCE_NEW_ROW: NEW                     CREATED: 2019-12-26 22:30:44.31                    SQL_MODE: ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_  FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION                     DEFINER: [email protected]        CHARACTER_SET_CLIENT: utf8mb4        COLLATION_CONNECTION: utf8mb4_general_ci          DATABASE_COLLATION: utf8mb4_0900_ai_ci  1 row in set (0.00 sec)  

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The result shows before_insert The database where the trigger is located 、 Data sheet 、 Trigger Name 、 Event type and triggered when the trigger is executed SQL Statement and other information .

When no condition is specified , Will query all trigger information .

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mysql> SELECT * FROM information_schema.triggers \G  *************************** 1. row ***************************             TRIGGER_CATALOG: def              TRIGGER_SCHEMA: sys                TRIGGER_NAME: sys_config_insert_set_user          EVENT_MANIPULATION: INSERT        EVENT_OBJECT_CATALOG: def         EVENT_OBJECT_SCHEMA: sys          EVENT_OBJECT_TABLE: sys_config                ACTION_ORDER: 1            ACTION_CONDITION: NULL            ACTION_STATEMENT: BEGIN      IF @sys.ignore_sys_config_triggers != true AND NEW.set_by IS NULL THEN          SET NEW.set_by = USER();      END IF;  END          ACTION_ORIENTATION: ROW               ACTION_TIMING: BEFORE  ACTION_REFERENCE_OLD_TABLE: NULL  ACTION_REFERENCE_NEW_TABLE: NULL    ACTION_REFERENCE_OLD_ROW: OLD    ACTION_REFERENCE_NEW_ROW: NEW                     CREATED: 2019-11-24 12:46:55.84                    SQL_MODE: ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_  FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION                     DEFINER: [email protected]        CHARACTER_SET_CLIENT: utf8mb4        COLLATION_CONNECTION: utf8mb4_0900_ai_ci          DATABASE_COLLATION: utf8mb4_0900_ai_ci  ############# Omit here n Row data #######################  *************************** 8. row ***************************             TRIGGER_CATALOG: def              TRIGGER_SCHEMA: goods                TRIGGER_NAME: after_delete          EVENT_MANIPULATION: DELETE        EVENT_OBJECT_CATALOG: def         EVENT_OBJECT_SCHEMA: goods          EVENT_OBJECT_TABLE: test_trigger                ACTION_ORDER: 1            ACTION_CONDITION: NULL            ACTION_STATEMENT: BEGIN  INSERT INTO test_trigger_log  (t_log)  VALUES  ('after_delete')  ;  END          ACTION_ORIENTATION: ROW               ACTION_TIMING: AFTER  ACTION_REFERENCE_OLD_TABLE: NULL  ACTION_REFERENCE_NEW_TABLE: NULL    ACTION_REFERENCE_OLD_ROW: OLD    ACTION_REFERENCE_NEW_ROW: NEW                     CREATED: 2019-12-26 23:35:19.34                    SQL_MODE: ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_  FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION                     DEFINER: [email protected]        CHARACTER_SET_CLIENT: utf8mb4        COLLATION_CONNECTION: utf8mb4_general_ci          DATABASE_COLLATION: utf8mb4_0900_ai_ci  8 rows in set (0.00 sec)  

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17.3　 Delete trigger

When confirming in the database that a trigger is no longer in use , You can delete triggers that are no longer used , stay MySQL in , Use DROP TRIGGER Statement delete trigger .

17.3.1　 Grammar format

The syntax format of delete trigger is as follows ：

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DROP TRIGGER [IF EXISTS] [schema_name.]trigger_name  

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The syntax format is described as follows ：

·DROP TRIGGER： Delete trigger's SQL keyword .

·schema_name： The name of the database where the trigger is located , It can be omitted . When omitted , Will delete MySQL Trigger under the database where the command line is located .

·trigger_name： Trigger name .

17.3.2　 Delete trigger example

for example , Delete the name as after_delete The trigger of .

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mysql> DROP TRIGGER after_delete;  Query OK, 0 rows affected (0.11 sec)  

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SQL Statement executed successfully , see alter_delete Trigger information .

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mysql> SHOW CREATE TRIGGER after_delete \G  ERROR 1360 (HY000): Trigger does not exist  

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Results show ,MySQL Report errors , non-existent alter_delete trigger , explain alter_delete The trigger has been successfully deleted .