mysql - 子查询返回多行..如何处理它？

Question

SELECT requestID
FROM request
WHERE userId = (
    SELECT userID
    FROM department
    WHERE desig = 'E'
    AND dept = (
        SELECT dept
        FROM department
        WHERE userId = it18
        AND desig = 'FM'
      )
);

Answer 1

使用以下方式编写此查询会更清晰JOIN：

select distinct r.requestID 
from 
    request r
    join department d
        on d.userId = r.userID
        and desig = 'E'
    join department d2
        on d2.dept = d.dept
        and d2.desig = 'FM'
        and d2.userId = 'it18'

或者，您可以简单地替换=为IN：

SELECT requestID
FROM request
WHERE userId IN (
    SELECT userID
    FROM department
    WHERE desig = 'E'
    AND dept IN (
        SELECT dept
        FROM department
        WHERE userId = it18
        AND desig = 'FM'
      )
);

它们应该返回相同的结果，但同时尝试两者以查看性能是否有任何差异。

Answer 2

有两种处理方法：

选项 1：更改为WHERE ... IN (SELECT ...)，如下所示：

SELECT requestID
FROM request
WHERE userId IN (
    SELECT userID
    FROM department
    WHERE desig = 'E'
    AND dept IN (
        SELECT dept
        FROM department
        WHERE userId = it18
        AND desig = 'FM'
      )
);

选项 2：使用聚合函数（如 ）仅强制一个结果MAX()，如下所示：

SELECT requestID
FROM request
WHERE userId = (
    SELECT MAX(userID)
    FROM department
    WHERE desig = 'E'
    AND dept = (
        SELECT MAX(dept)
        FROM department
        WHERE userId = it18
        AND desig = 'FM'
      )
);