group by连接1个字段：
1.select后接group by后面的字段
2.顺序：where----group by-----order by------having
- select 后面的所有列中,没有使用聚合函数的列,必须出现在 group by 后面
group by 后连接2个字段：
一个字段就依据一个条件分组，两个字段就依据两个条件分组。。。。。。
不满足另外分为1组。
力扣刷题：1050合作过至少三次的演员和导演
ActorDirector 表：
+-------------+---------+
| Column Name | Type |
+-------------+---------+
| actor_id | int |
| director_id | int |
| timestamp | int |
+-------------+---------+
在 SQL 中，timestamp 是这张表的主键.
查询合作过至少三次的演员和导演的 id 对 (actor_id, director_id）
答案：
sql
select actor_id,director_id from actordirector
group by actor_id,director_id
having count(*)>=3表结构：
sql
Create table If Not Exists ActorDirector (actor_id int, director_id int, timestamp int)
Truncate table ActorDirector
insert into ActorDirector (actor_id, director_id, timestamp) values ('1', '1', '0')
insert into ActorDirector (actor_id, director_id, timestamp) values ('1', '1', '1')
insert into ActorDirector (actor_id, director_id, timestamp) values ('1', '1', '2')
insert into ActorDirector (actor_id, director_id, timestamp) values ('1', '2', '3')
insert into ActorDirector (actor_id, director_id, timestamp) values ('1', '2', '4')
insert into ActorDirector (actor_id, director_id, timestamp) values ('2', '1', '5')
insert into ActorDirector (actor_id, director_id, timestamp) values ('2', '1', '6')