SQL – group, having
group
truncate table user1; insert into user1 (id, username) values (1, ‘test1‘); insert into user1 (id, username) values (2, ‘test1‘); insert into user1 (id, username) values (3, ‘test1‘); insert into user1 (id, username) values (4, ‘test1‘); insert into user1 (id, username) values (5, ‘test1‘); insert into user1 (id, username) values (1, ‘test2‘); insert into user1 (id, username) values (2, ‘test2‘); insert into user1 (id, username) values (3, ‘test2‘); insert into user1 (id, username) values (4, ‘test2‘); insert into user1 (id, username) values (5, ‘test2‘); insert into user1 (id, username) values (1, ‘test3‘); insert into user1 (id, username) values (2, ‘test3‘); insert into user1 (id, username) values (3, ‘test3‘); insert into user1 (id, username) values (4, ‘test3‘); insert into user1 (id, username) values (5, ‘test3‘);
select * from user1 group by username;
| id | username | +----+----------+ | 1 | test1 | | 1 | test2 | | 1 | test3 |
select * from user1 group by id;
| id | username | +----+----------+ | 1 | test1 | | 2 | test1 | | 3 | test1 | | 4 | test1 | | 5 | test1 |
分组实际上就是求集合?
select username, id from user1 group by 1;
1 指的是 username, group by 2 指的是 id (一般不这样用, 好丑陋的用法)
having
相当于 where
select username, id from user1 group by id;
| username | id | +----------+----+ | test1 | 1 | | test1 | 2 | | test1 | 3 | | test1 | 4 | | test1 | 5 |
select username, id from user1 group by id having id in (1, 2, 3);
| username | id | +----------+----+ | test1 | 1 | | test1 | 2 | | test1 | 3 |
select username from user1 group by id having id in (1, 2, 3);
select username from user1 group by username having id in (1, 2, 3); # 报错, 没有 id 这个字段
