MySQL选择上一个语句的结果

Question

我对SQL很陌生我遇到了这个问题： 我有人有权力的关系表

A has X
A has Z
B has X
B has Y
C has Y
D has Z

我想选择所有具有X和Y权力的人，在这种情况下是B.当然，我将拥有更复杂的权力组合。

Answer 1

做这样的事，

SELECT person
FROM tableName
WHERE power IN ('x', 'y')
GROUP BY Person
HAVING COUNT(*) = 2

• SQLFiddle Demo Link

如果对每个人的power强制执行唯一约束，则需要使用DISTINCT

SELECT person
FROM tableName
WHERE power IN ('x', 'y')
GROUP BY Person
HAVING COUNT(DISTINCT power) = 2

• SQLFiddle Demo Link (with duplicate)

Answer 2

这称为relational division，这是执行此操作的一种方法：

SELECT person
FROM Powers
WHERE power IN ('x', 'y')
GROUP BY person
HAVING COUNT(power) = 2