sql example 9 – group, having

• group
  
  • sql create
    
  • sql delete
    
  • sql select
    

  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;

• 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