[Leetcode 1050] Actors and Directors Who Cooperated At Least Three Times

原题说明

Table: ActorDirector

+————-+———+
| Column Name | Type    |
+————-+———+
| actor_id    | int     |
| director_id | int     |
| timestamp   | int     |
+————-+———+
timestamp is the primary key column for this table.

Write a SQL query for a report that provides the pairs (actor_id, director_id) where the actor have cooperated with the director at least 3 times.

Example:

ActorDirector table:
+————-+————-+————-+
| actor_id    | director_id | timestamp   |
+————-+————-+————-+
| 1           | 1           | 0           |
| 1           | 1           | 1           |
| 1           | 1           | 2           |
| 1           | 2           | 3           |
| 1           | 2           | 4           |
| 2           | 1           | 5           |
| 2           | 1           | 6           |
+————-+————-+————-+

Result table:
+————-+————-+
| actor_id | director_id |
+————-+————-+
| 1 | 1 |
+————-+————-+
The only pair is (1, 1) where they cooperated exactly 3 times.

解题思路

我们需要找出一起至少合作过3次的演员和导演。

对二元组(演员,导演)分组，分别计算每组个数，选出每组个数>=3的行。
这里我们需要应用group by语法和count函数。

示例代码 (mysql)

SELECT A.actor_id, A.director_id
FROM ActorDirector AS A
GROUP BY A.actor_id, A.director_id
HAVING COUNT(*) >= 3;

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