oracle里面可以通过row_number() OVER (PARTITION BY cid,author ORDER BY id DESC) 表示根据cid,author分组，在分组内部根据id排序，而此函数计算的值就表示每组内部排序后的顺序编号（组内连续的唯一的)，而mysql数据库就没有这样的统计函数，需要自己写复杂的sql来实现。
1，录入测试数据
USE csdn;
DROPTABLEIFEXISTS test;
CREATETABLE test (
id INT PRIMARYKEY,
cid INT,
author VARCHAR(30)
) ENGINE=INNODB;
INSERTINTO 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\‘);
INSERTINTO test VALUES(200,200,\‘200test_nagios\‘);
2，原始的效率比较低下的子查询实现方式 SQL代码如下：
SELECT*FROM test a
WHERE
N>(
SELECTCOUNT(*)
FROM test b
WHERE a.cid=b.cid AND a.`author`=b.`author` AND a.id<b.id
)ORDERBY cid,author,id DESC;
只要将N换成你要的数字比如2，就表示查询出每个分组的前2条记录，如下所示：
mysql>SELECT*FROM test a
->WHERE
-> 2>(
->SELECTCOUNT(*)
->FROM test b
->WHERE a.cid=b.cid AND a.`author`=b.`author` AND a.id<b.id
->)ORDERBY cid,author,id DESC;
+-----+------+----------------+
| id | cid | author |
+-----+------+----------------+
| 2 | 1 | test1 |
| 1 | 1 | test1 |
| 5 | 1 | test2 |
| 4 | 1 | test2 |
| 9 | 1 | test3 |
| 8 | 1 | test3 |
| 11 | 2 | test11 |
| 10 | 2 | test11 |
| 14 | 2 | test22 |
| 13 | 2 | test22 |
| 18 | 2 | test33 |
| 17 | 2 | test33 |
| 200 | 200 | 200test_nagios |
+-----+------+----------------+
13 ROWSINSET(0.00 sec)
mysql>
3，使用动态sql来实现 先构造序列号码，引入一个@row来做rownumber SET @row=0;SET @mid=‘‘;SELECT cid, author, @row:=@row+1 rownum FROM test ORDER BY cid, author LIMIT 10;
序列号码已经出来了，再加一个@mid来进行分组，重点在于CASE WHEN @mid = author THEN @row:=@row+1 ELSE @row:=1 END rownum，表示分组的时候会自动从1计数指导这个分组数据遍历结束。 SET @row=0;SET @mid=‘‘;SELECT cid, author,CASE WHEN @mid = author THEN @row:=@row+1 ELSE @row:=1 END rownum, @mid:=author FROM test ORDER BY cid,author DESC LIMIT 20;
好了，再外面加一层inner JOIN 再对 rownumber 做限制 就可以拿到目标数据了。 SET @row=0; SET @mid=‘‘; SELECT a.*,b.rownum FROM test a INNER JOIN ( SELECT cid, author, id, CASE WHEN @mid = author THEN @row:=@row+1 ELSE @row:=1 END rownum, @mid:=author MID FROM test ORDER BY cid,author,id DESC ) b ON b.author=a.author AND b.cid=a.cid AND b.id=a.id WHERE b.rownum<3;
执行结果如下所示：
mysql>SET@row=0;
QUERY OK, 0 ROWS affected (0.00 sec)
mysql>SET@mid=\‘\‘;
QUERY OK, 0 ROWS affected (0.00 sec)
mysql>SELECT a.*,b.rownumFROM test a
-> INNER JOIN (
->SELECT cid, author, id, CASE WHEN @mid = author THEN@row:=@row+1 ELSE@row:=1 ENDrownum,@mid:=author MID
->FROM test
->ORDERBY cid,author,id DESC
->) b ON b.author=a.author AND b.cid=a.cid AND b.id=a.id WHERE b.rownum<3;
