问题描述
是否可以根据 MySQL 中的查询结果设置用户变量?
Is it possible to set an user variable based on the result of a query in MySQL?
我想要实现的是这样的(我们可以假设 USER 和 GROUP 都是唯一的):
What I want to achieve is something like this (we can assume that both USER and GROUP are unique):
set @user = 123456;
set @group = select GROUP from USER where User = @user;
select * from USER where GROUP = @group;
请注意，我知道这是可能的，但我不希望使用嵌套查询来做到这一点.
Please note that I know it's possible but I do not wish to do this with nested queries.
推荐答案
可以，但是需要将变量赋值移到查询中:
Yes, but you need to move the variable assignment into the query:
SET @user := 123456;
SELECT @group := `group` FROM user WHERE user = @user;
SELECT * FROM user WHERE `group` = @group;
测试用例:
CREATE TABLE user (`user` int, `group` int);
INSERT INTO user VALUES (123456, 5);
INSERT INTO user VALUES (111111, 5);
结果:
SET @user := 123456;
SELECT @group := `group` FROM user WHERE user = @user;
SELECT * FROM user WHERE `group` = @group;
+--------+-------+
| user | group |
+--------+-------+
| 123456 | 5 |
| 111111 | 5 |
+--------+-------+
2 rows in set (0.00 sec)
请注意，对于 SET，= 或 := 都可以用作赋值运算符.但是在其他语句中，赋值运算符必须是 := 而不是 = 因为 = 在非 SET 语句中被视为比较运算符.
Note that for SET, either = or := can be used as the assignment operator. However inside other statements, the assignment operator must be := and not = because = is treated as a comparison operator in non-SET statements.
更新:
除了下面的评论，您还可以执行以下操作:
Further to comments below, you may also do the following:
SET @user := 123456;
SELECT `group` FROM user LIMIT 1 INTO @group;
SELECT * FROM user WHERE `group` = @group;
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