MySQL SELECT如果仅匹配一个值

Question

我的要求似乎并不那么困难,但我不确定最好的方法.

我有下表:

userID  file
   1      1
   1      2 
   1      3
   2      1
   2      3
   3      2
   4      1
   4      2

我想选择只有文件号2的UserID .在我的例子中,结果只有3.

Answer 1

SELECT userID
FROM tableName a
WHERE file = 2
GROUP BY userID
HAVING COUNT(*) = 
(
  SELECT COUNT(*)
  FROM tableName b
  WHERE a.userID = b.userID 
)

• SQLFiddle演示