MySQL gets the top 1 and top n records after grouping

Sometimes there are some needs , After query grouping The maximum of , The whole row of records or grouped records where the minimum value is located top n The record of the line , In some other databases, there may be window functions that can be found in various aspects , however MySQL Without these functions , There is no direct way to find out , You can query through the following methods .

preparation

The structure of the test table is as follows ：

root:test> show create table test1\G
*************************** 1. row ***************************
       Table: test1
Create Table: CREATE TABLE `test1` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(20) DEFAULT NULL,
  `course` varchar(20) DEFAULT NULL,
  `score` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

  insert data ：

insert into test1(name,course,score)
values
(' Zhang San ',' Chinese language and literature ',80),
(' Li Si ',' Chinese language and literature ',90),
(' Wang Wu ',' Chinese language and literature ',93),
(' Zhang San ',' mathematics ',77),
(' Li Si ',' mathematics ',68),
(' Wang Wu ',' mathematics ',99),
(' Zhang San ',' English ',90),
(' Li Si ',' English ',50),
(' Wang Wu ',' English ',89);

View results ：

root:test>  select * from test1;
+----+--------+--------+-------+
| id | name   | course | score |
+----+--------+--------+-------+
|  1 |  Zhang San    |  Chinese language and literature    |    80 |
|  2 |  Li Si    |  Chinese language and literature    |    90 |
|  3 |  Wang Wu    |  Chinese language and literature    |    93 |
|  4 |  Zhang San    |  mathematics    |    77 |
|  5 |  Li Si    |  mathematics    |    68 |
|  6 |  Wang Wu    |  mathematics    |    99 |
|  7 |  Zhang San    |  English    |    90 |
|  8 |  Li Si    |  English    |    50 |
|  9 |  Wang Wu    |  English    |    89 |
+----+--------+--------+-------+

TOP 1

Check each course The highest score Students and grades

1、 Use self connect 【 recommend 】

root:test> select a.name,a.course,a.score from
    -> test1 a
    -> join (select course,max(score) score from test1 group by course) b 
    -> on a.course=b.course and a.score=b.score;
+--------+--------+-------+
| name   | course | score |
+--------+--------+-------+
|  Wang Wu    |  Chinese language and literature    |    93 |
|  Wang Wu    |  mathematics    |    99 |
|  Zhang San    |  English    |    90 |
+--------+--------+-------+
3 rows in set (0.00 sec)

2、 Use related subqueries

root:test> select name,course,score from test1 a
    -> where score=(select max(score) from test1 where a.course=test1.course);
+--------+--------+-------+
| name   | course | score |
+--------+--------+-------+
|  Wang Wu    |  Chinese language and literature    |    93 |
|  Wang Wu    |  mathematics    |    99 |
|  Zhang San    |  English    |    90 |
+--------+--------+-------+
3 rows in set (0.00 sec)

perhaps

root:test> select name,course,score from test1 a
    -> where not exists(select 1 from test1 where a.course=test1.course and a.score < test1.score);
+--------+--------+-------+
| name   | course | score |
+--------+--------+-------+
|  Wang Wu    |  Chinese language and literature    |    93 |
|  Wang Wu    |  mathematics    |    99 |
|  Zhang San    |  English    |    90 |
+--------+--------+-------+
3 rows in set (0.00 sec)

TOP N

N>=1

Check the top two students of each course and their grades

1、 Use union all

If the result set is small , You can use the program to query a single grouping result and then piece it up , You can also use union all

root:test> (select name,course,score from test1 where course=' Chinese language and literature ' order by score desc limit 2)
    -> union all
    -> (select name,course,score from test1 where course=' mathematics ' order by score desc limit 2)
    -> union all
    -> (select name,course,score from test1 where course=' English ' order by score desc limit 2);
+--------+--------+-------+
| name   | course | score |
+--------+--------+-------+
|  Wang Wu    |  Chinese language and literature    |    93 |
|  Li Si    |  Chinese language and literature    |    90 |
|  Wang Wu    |  mathematics    |    99 |
|  Zhang San    |  mathematics    |    77 |
|  Zhang San    |  English    |    90 |
|  Wang Wu    |  English    |    89 |
+--------+--------+-------+
6 rows in set (0.01 sec)

2、 Self left connection

root:test> select a.name,a.course,a.score
    -> from test1 a left join test1 b on a.course=b.course and a.score<b.score
    -> group by a.name,a.course,a.score
    -> having count(b.id)<2
    -> order by a.course,a.score desc;
+--------+--------+-------+
| name   | course | score |
+--------+--------+-------+
|  Wang Wu    |  mathematics    |    99 |
|  Zhang San    |  mathematics    |    77 |
|  Zhang San    |  English    |    90 |
|  Wang Wu    |  English    |    89 |
|  Wang Wu    |  Chinese language and literature    |    93 |
|  Li Si    |  Chinese language and literature    |    90 |
+--------+--------+-------+
6 rows in set (0.00 sec)

3、 Correlation subquery

root:test> select *
    -> from test1 a
    -> where 2>(select count(*) from test1 where course=a.course and score>a.score)
    -> order by a.course,a.score desc;
+----+--------+--------+-------+
| id | name   | course | score |
+----+--------+--------+-------+
|  6 |  Wang Wu    |  mathematics    |    99 |
|  4 |  Zhang San    |  mathematics    |    77 |
|  7 |  Zhang San    |  English    |    90 |
|  9 |  Wang Wu    |  English    |    89 |
|  3 |  Wang Wu    |  Chinese language and literature    |    93 |
|  2 |  Li Si    |  Chinese language and literature    |    90 |
+----+--------+--------+-------+
6 rows in set (0.01 sec)

4、 Use user variables

root:test> set @num := 0, @course := '';
Query OK, 0 rows affected (0.00 sec)

root:test> 
root:test> select name, course, score
    -> from (
    ->    select name, course, score,
    ->       @num := if(@course = course, @num + 1, 1) as row_number,
    ->       @course := course as dummy
    ->   from test1
    ->   order by course, score desc
    -> ) as x where x.row_number <= 2;
+--------+--------+-------+
| name   | course | score |
+--------+--------+-------+
|  Wang Wu    |  mathematics    |    99 |
|  Zhang San    |  mathematics    |    77 |
|  Zhang San    |  English    |    90 |
|  Wang Wu    |  English    |    89 |
|  Wang Wu    |  Chinese language and literature    |    93 |
|  Li Si    |  Chinese language and literature    |    90 |
+--------+--------+-------+
6 rows in set (0.00 sec)