mysql - MySQL 选择上一条语句的结果

Question

我对 SQL 很陌生，我遇到了这个问题：我有关系表，其中人有权力

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，我会有一个更复杂的权力组合。

Answer 1

做这样的事情，

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

• SQLFiddle 演示链接

如果没有对每个人强制执行唯一约束power，则需要使用DISTINCT

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

• SQLFiddle 演示链接（有重复）

Answer 2

这称为关系除法，这是一种方法：

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