如何根据表中的特定条件选择表中的特定行？

Question

我一整天都在思考这个问题，我似乎无法想出一个解决方案。让我们做一个例子，我有一个表，看起来像这样：

| movies | personname | personrole | 
------------------------------------- 
| A  | p1   | director | 
| A  | p1   | producer | 
| A  | p1   | writer  | 
| A  | p1   | actor  | 
| A  | p2   | actor  | 
| A  | p3   | writer  | 
| A  | p3   | producer | 
| B  | p4   | director | 
| B  | p4   | producer | 
| B  | p4   | writer  | 

现在，我想要做的是列出所有的人在这个表是一个导演，制片人，并为电影作家。任何帮助表示赞赏！

This as output: 

| movies | personname | 
----------------------- 
| A  | p1   | 
| B  | p4   | 

Answer 1

此问题被称为SQL of Relational Division。

SELECT movies, 
     personName 
FROM tableName 
WHERE personrole IN ('director','producer','writer') 
GROUP BY movies, personName 
HAVING COUNT(*) = 3 

如果personrole不是每个movies独一无二的，personname，则需要使用DISTINCT

SELECT movies, 
     personName 
FROM tableName 
WHERE personrole IN ('director','producer','writer') 
GROUP BY movies, personName 
HAVING COUNT(DISTINCT personrole) = 3 

• 计算唯一值SQLFiddle Demo (both queries)