问题描述

我有一个触发器，其中我想要一个变量来保存从 SELECT 获得的 INT，因此我可以在两个 IF 语句中使用它，而不是调用 SELECT 两次.MySQL触发器中如何声明/使用变量?

I have a trigger in which I want to have a variable that holds an INT I get from a SELECT, so I can use it in two IF statements instead of calling the SELECT twice. How do you declare/use variables in MySQL triggers?

推荐答案

您可以使用 DECLARE 语法在 MySQL 触发器中声明局部变量.

You can declare local variables in MySQL triggers, with the DECLARE syntax.

这是一个例子:

DROP TABLE IF EXISTS foo;
CREATE TABLE FOO (
  i SERIAL PRIMARY KEY
);

DELIMITER //
DROP TRIGGER IF EXISTS bar //

CREATE TRIGGER bar AFTER INSERT ON foo
FOR EACH ROW BEGIN
  DECLARE x INT;
  SET x = NEW.i;
  SET @a = x; -- set user variable outside trigger
END//

DELIMITER ;

SET @a = 0;

SELECT @a; -- returns 0

INSERT INTO foo () VALUES ();

SELECT @a; -- returns 1, the value it got during the trigger

为变量赋值时，必须确保查询仅返回单个值，而不是一组行或一组列.例如，如果您的查询在实践中返回单个值，那没关系，但是一旦它返回多于一行，您就会得到ERROR 1242: Subquery returns more than 1 row".

When you assign a value to a variable, you must ensure that the query returns only a single value, not a set of rows or a set of columns. For instance, if your query returns a single value in practice, it's okay but as soon as it returns more than one row, you get "ERROR 1242: Subquery returns more than 1 row".

您可以使用 LIMIT 或 MAX() 来确保将局部变量设置为单个值.

You can use LIMIT or MAX() to make sure that the local variable is set to a single value.

CREATE TRIGGER bar AFTER INSERT ON foo
FOR EACH ROW BEGIN
  DECLARE x INT;
  SET x = (SELECT age FROM users WHERE name = 'Bill'); 
  -- ERROR 1242 if more than one row with 'Bill'
END//

CREATE TRIGGER bar AFTER INSERT ON foo
FOR EACH ROW BEGIN
  DECLARE x INT;
  SET x = (SELECT MAX(age) FROM users WHERE name = 'Bill');
  -- OK even when more than one row with 'Bill'
END//

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