表结构和数据
create table cat(
id(1) int not null auto_increment primary key,
cat_id int(1),
value int(1),
name varchar(20)
);
insert into cat (cat_id,name,value) values ('1','name1', '1');
insert into cat (cat_id,name,value) values ('1','name2', '2');
insert into cat (cat_id,name,value) values ('1','name3', '3');
insert into cat (cat_id,name,value) values ('1','name4', '4');
insert into cat (cat_id,name,value) values ('2','name5', '5');
insert into cat (cat_id,name,value) values ('2','name6', '6');
insert into cat (cat_id,name,value) values ('2','name7', '7');
insert into cat (cat_id,name,value) values ('2','name8', '8');
insert into cat (cat_id,name,value) values ('3','name9', '9');
insert into cat (cat_id,name,value) values ('3','name10','10');
insert into cat (cat_id,name,value) values ('3','name11','11');
insert into cat (cat_id,name,value) values ('3','name12','12');
mysql> select *from cat;
+----+--------+-------+--------+
| id | cat_id | value | name |
+----+--------+-------+--------+
| 1 | 1 | 1 | name1 |
| 2 | 1 | 2 | name2 |
| 3 | 1 | 3 | name3 |
| 4 | 1 | 4 | name4 |
| 5 | 2 | 5 | name5 |
| 6 | 2 | 6 | name6 |
| 7 | 2 | 7 | name7 |
| 8 | 2 | 8 | name8 |
| 9 | 3 | 9 | name9 |
| 10 | 3 | 10 | name10 |
| 11 | 3 | 11 | name11 |
| 12 | 3 | 12 | name12 |
+----+--------+-------+--------+
12 rows in set (0.13 sec)
查询分组最大记录
// 默认取分组第一条
mysql> select *from cat group by cat_id order by cat_id;
+----+--------+-------+-------+
| id | cat_id | value | name |
+----+--------+-------+-------+
| 1 | 1 | 1 | name1 |
| 5 | 2 | 5 | name5 |
| 9 | 3 | 9 | name9 |
+----+--------+-------+-------+
3 rows in set (0.00 sec)
mysql> select *from (select *from cat order by value desc) a group by cat_id;
+----+--------+-------+--------+
| id | cat_id | value | name |
+----+--------+-------+--------+
| 4 | 1 | 4 | name4 |
| 8 | 2 | 8 | name8 |
| 12 | 3 | 12 | name12 |
+----+--------+-------+--------+
3 rows in set (0.06 sec)
mysql> select a.* from cat a where value = (select max(value) from cat where cat
_id = a.cat_id) order by a.cat_id;
mysql> select a.* from cat a,(select cat_id,max(value) value from cat group by cat_id) b where a.cat_id = b.cat_id and a.value = b.value order by a.cat_id;
mysql> select a.* from cat a inner join (select cat_id, max(value) value from cat group by cat_id) b on a.cat_id= b.cat_id and a.value= b.value order by a.cat_id;
+----+--------+-------+--------+
| id | cat_id | value | name |
+----+--------+-------+--------+
| 4 | 1 | 4 | name4 |
| 8 | 2 | 8 | name8 |
| 12 | 3 | 12 | name12 |
+----+--------+-------+--------+
3 rows in set (0.00 sec)
分组前 3 条记录
mysql> select a.* from cat a where exists (select count(*) from cat where cat_id= a.cat_id and value > a.value having Count(*) < 3) order by a.cat_id,a.value desc;
mysql> select *from cat a where (select count(*) from cat b where a.cat_id=b.cat_id and b.value>a.value) < 3 order by a.cat_id,a.value desc;
+----+--------+-------+--------+
| id | cat_id | value | name |
+----+--------+-------+--------+
| 4 | 1 | 4 | name4 |
| 3 | 1 | 3 | name3 |
| 2 | 1 | 2 | name2 |
| 8 | 2 | 8 | name8 |
| 7 | 2 | 7 | name7 |
| 6 | 2 | 6 | name6 |
| 12 | 3 | 12 | name12 |
| 11 | 3 | 11 | name11 |
| 10 | 3 | 10 | name10 |
+----+--------+-------+--------+
9 rows in set (0.15 sec)