MySQL存储过程 handler用法

DECLARE ... HANDLER语法

参考官方手册：https://dev.mysql.com/doc/refman/5.7/en/declare-handler.html

DECLARE handler_action HANDLER
    FOR condition_value [, condition_value] ...
    statement

handler_action: {
    CONTINUE
  | EXIT
  | UNDO
}

condition_value: {
    mysql_error_code
  | SQLSTATE [VALUE] sqlstate_value
  | condition_name
  | SQLWARNING
  | NOT FOUND
  | SQLEXCEPTION
}

官方手册sqlstate与error code参考: https://dev.mysql.com/doc/refman/8.0/en/server-error-reference.html

CONTINUE 与 EXIT 对比

CONTINUE: 发送错误时继续执行后续代码
EXIT: 发生错误时退出当前代码块（可能是子代码块或者main代码块）

直接看例子

CONTINUE

mysql> CREATE TABLE d.handlerdemo1 (s1 INT, PRIMARY KEY (s1));
Query OK, 0 rows affected (0.00 sec)

mysql> delimiter //

mysql> CREATE PROCEDURE handlerdemo ()
       BEGIN
         DECLARE CONTINUE HANDLER FOR SQLSTATE '23000' SET @y = 1;
         
         TRUNCATE TABLE d.handlerdemo1;

         SET @x = 1;
         INSERT INTO d.handlerdemo1 VALUES (1);

         SET @x = 2;
         INSERT INTO d.handlerdemo1 VALUES (1);

         SET @x = 3;
       END;
       //
Query OK, 0 rows affected (0.00 sec)

mysql> CALL handlerdemo()//
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @x//
    +------+
    | @x   |
    +------+
    | 3    |
    +------+
    1 row in set (0.00 sec)

注意@x是3在过程执行之后，这表明执行继续该过程的结束发生错误之后。如果该 DECLARE ... HANDLER语句不存在，MySQL会因为约束而EXIT在第二次INSERT失败后采取默认的action（）PRIMARY KEY，并且 SELECT @x会返回 2。

来看一下EXIT

mysql> show create procedure handlerdemo1\G
*************************** 1. row ***************************
           Procedure: handlerdemo1
            sql_mode: STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION
    Create Procedure: CREATE DEFINER=`root`@`localhost` PROCEDURE `handlerdemo1`()
BEGIN
　DECLARE EXIT HANDLER FOR SQLSTATE '23000'SET @y = 1;
　TRUNCATE TABLE d.handlerdemo1;

　SET @x = 1;
　INSERT INTO d.handlerdemo1 VALUES (1);

　SET @x = 2;
　INSERT INTO d.handlerdemo1 VALUES (1);

　SET @x = 3;
END
character_set_client: utf8mb4
collation_connection: utf8mb4_0900_ai_ci
  Database Collation: latin1_swedish_ci
1 row in set (0.00 sec)

mysql> call handlerdemo1();
Query OK, 0 rows affected (0.26 sec)

mysql> SELECT @x;
+------+
| @x   |
+------+
|    2 |
+------+
1 row in set (0.00 sec)

SELECT @x 的确返回 2 。