MySQL分组排序取前N条记录以及生成自动数字序列的SQL

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博客介绍了MySQL分组排序取前N条记录的最简洁单条sql。包含创建表、插入数据的操作，给出了分组排序的查询语句，还说明了N的含义，即取分组后前面几个的判断依据，最后展示了生成自动数字序列的操作。

-- MySQL分组排序取前N条记录的最简洁的单条sql。

USE test;

DROP TABLE IF EXISTS test;

CREATE TABLE test (
id INT PRIMARY KEY,
cid INT,
author VARCHAR(30)
) ENGINE=MYISAM;

INSERT INTO test VALUES
(1,1,'test1'),
(2,1,'test1'),
(3,1,'test2'),
(4,1,'test2'),
(5,1,'test2'),
(6,1,'test3'),
(7,1,'test3'),
(8,1,'test3'),
(9,1,'test3'),
(10,2,'test11'),
(11,2,'test11'),
(12,2,'test22'),
(13,2,'test22'),
(14,2,'test22'),
(15,2,'test33'),
(16,2,'test33'),
(17,2,'test33'),
(18,2,'test33');

INSERT INTO test VALUES (200,200,'200test_nagios');

SELECT * FROM (SELECT cid,author,COUNT(*) AS number FROM test GROUP BY cid,author) a
WHERE
N>(
SELECT COUNT(*)
FROM (SELECT cid,author,COUNT(*) AS number FROM test GROUP BY cid,author) b
WHERE a.cid=b.cid AND a.number<b.number
)ORDER BY cid,number DESC;

结果如下：

mysql> SELECT * FROM (SELECT cid,author,COUNT(*) AS number FROM test GROUP BY cid,author) a 
    -> WHERE  
    -> 3>(
    ->     SELECT COUNT(*) 
    ->     FROM (SELECT cid,author,COUNT(*) AS number FROM test GROUP BY cid,author) b
    ->     WHERE a.cid=b.cid AND a.number<b.number
    -> )ORDER BY cid,number DESC;
+------+----------------+--------+
| cid  | author         | number |
+------+----------------+--------+
|    1 | test3          |      4 |
|    1 | test2          |      3 |
|    1 | test1          |      2 |
|    2 | test33         |      4 |
|    2 | test22         |      3 |
|    2 | test11         |      2 |
|  200 | 200test_nagios |      1 |
+------+----------------+--------+
7 rows in set (0.00 sec)

N就是取分组之后的最前面几个判断，N=3就是取前3个

-- 生成自动数字序列
SET @ROW=0;
SELECT a.*,(@ROW:=@ROW +1)Rank
FROM test a;