MySQL 错误 [1241] [21000]: Operand should contain 1 column(s)

Operand should contain 1 column(s)可能有很多原因，记录一下自己碰到的情况

1. where子查询中select目标太多

直接上例子
错误：

SELECT 
	student.sId ,student.sName 
FROM 
	student 
WHERE   student.sId
NOT IN (SELECT sc.sId , count(sc.sId) AS count_sid FROM sc GROUP BY sc.sId HAVING count_sid =3)

正确

SELECT 
	student.sId ,student.sName 
FROM 
	student 
WHERE   student.sId
NOT IN (SELECT sc.sId FROM sc GROUP BY sc.sId HAVING count(sc.sId) =3)