前言：

????????同事的業務場景是，按照cid、author分組，再按照id倒敘，取出前2條記錄出來。

????????oracle里面可以通過row_number() OVER (PARTITION BY cid,author ORDER BY id DESC) 表示根據cid,author分組，在分組內部根據id排序，而此函數計算的值就表示每組內部排序后的順序編號（組內連續的唯一的)，而mysql數據庫就沒有這樣的統計函數，需要自己寫復雜的sql來實現。


1，錄入測試數據

USE csdn;

DROP?TABLE?IF?EXISTS?test;

CREATE?TABLE?test?(

??id INT?PRIMARY?KEY,

??cid INT,

??author?VARCHAR(30)

)?ENGINE=INNODB;

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\');

2，原始的效率比較低下的子查詢實現方式
SQL代碼如下：

SELECT?*?FROM?test a?

WHERE?

N>(

????SELECT?COUNT(*)?

????FROM?test b

????WHERE?a.cid=b.cid?AND?a.`author`=b.`author`?AND?a.id<b.id

)ORDER?BY?cid,author,id?DESC;

只要將N換成你要的數字比如2，就表示查詢出每個分組的前2條記錄，如下所示：

mysql>?SELECT?*?FROM?test a?

????->?WHERE?

????->?2>(

????->?SELECT?COUNT(*)?

????->?FROM?test b

????->?WHERE?a.cid=b.cid?AND?a.`author`=b.`author`?AND?a.id<b.id

????->?)ORDER?BY?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?ROWS?IN?SET?(0.00 sec)

mysql>

3，使用動態sql來實現
先構造序列號碼，引入一個@row來做rownumber
SET @row=0;SET @mid='';SELECT cid, author, @row:=@row+1 rownum FROM test ORDER BY ?cid, author LIMIT 10; ??

序列號碼已經出來了，再加一個@mid來進行分組，重點在于CASE WHEN @mid = author THEN @row:=@row+1 ELSE @row:=1 END rownum，表示分組的時候會自動從1計數指導這個分組數據遍歷結束。
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，再外面加一層inner JOIN 再對 rownumber 做限制 就可以拿到目標數據了。
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; ?


執行結果如下所示：

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.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;?

+-----+------+----------------+--------+

|?id?|?cid?|?author?|?rownum?|

+-----+------+----------------+--------+

|?2?|?1?|?test1?|?1?|

|?1?|?1?|?test1?|?2?|

|?5?|?1?|?test2?|?1?|

|?4?|?1?|?test2?|?2?|

|?9?|?1?|?test3?|?1?|

|?8?|?1?|?test3?|?2?|

|?11?|?2?|?test11?|?1?|

|?10?|?2?|?test11?|?2?|

|?14?|?2?|?test22?|?1?|

|?13?|?2?|?test22?|?2?|

|?18?|?2?|?test33?|?1?|

|?17?|?2?|?test33?|?2?|

|?200?|?200?|?200test_nagios?|?1?|

+-----+------+----------------+--------+

13?ROWS?IN?SET?(0.01 sec)

mysql>

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