子查詢返回多行..如何處理它？

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

這將是更加清晰編寫使用JOIN此查詢：

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，你可以簡單地更換=與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' 
    ) 
); 

他們應該返回相同的結果，但嘗試以查看性能是否有差異。

Answer 2

有對付它的方法有兩種：

選項1：更改爲WHERE ... IN (SELECT ...)，像這樣：

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' 
    ) 
); 

選項2：只強制一個結果通過使用聚合函數一樣MAX()，像這：

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' 
    ) 
);