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

这将是更加清晰编写使用JOIN此查询：

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，你可以简单地更换=与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' 
    ) 
); 

他们应该返回相同的结果，但尝试以查看性能是否有差异。

Answer 2

有对付它的方法有两种：

选项1：更改为WHERE ... IN (SELECT ...)，像这样：

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：只强制一个结果通过使用聚合函数一样MAX()，像这：

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' 
    ) 
);