每个类别的MySQL限制结果

Question

c_data中的每个项目都在一个类别/部分中。我想限制每个类别显示多少项目，而不是限制检索项目的总数。很显然，如果我在查询中添加“限制20”之类的内容，它只会总共获取20个结果，而不是每个类别的20个结果。

SELECT cm.id, 
       cm.title AS cmtitle, 
       cm.sectionid, 
       cm.type AS cmtype, 
       cd.id, 
       cd.time, 
       cd.link, 
       cd.title, 
       cd.description, 
       cd.sectionid AS sectionid 
     FROM c_main AS cm 
     JOIN c_data AS cd ON cd.sectionid=cm.sectionid 
    WHERE cd.sectionid=cm.sectionid 
    ORDER by id ASC 

该分类的字段是“sectionid”。

Answer 1

MySQL没有任何等级的功能，但你可以使用一个变量来创建一个伪行号。

用途：

SELECT x.* 
    FROM (SELECT cm.id, 
       cm.title AS cmtitle, 
       cm.sectionid, 
       cm.type AS cmtype, 
       cd.id AS cd_id, 
       cd.time, 
       cd.link, 
       cd.title, 
       cd.description, 
       cd.sectionid AS cd_sectionid, 
       CASE 
       WHEN @sectionid != cm.sectionid THEN @rownum := 1 
       ELSE @rownum := @rownum + 1 
       END AS rank, 
       @sectionid := cm.sectionid 
      FROM C_MAIN cm, 
       C_DATA cd, 
       (SELECT @rownum := 0, @sectionid := NULL) r 
     WHERE cm.sectionid = cd.sectionid 
     ORDER BY cm.sectionid) x 
WHERE x.rank <= 20 
ORDER BY id 

Answer 2

这个previous post的答案应该可以帮助你解决这个问题。

编辑：

应该用row numbers工作。

我还没有尝试过，但是这应该工作：

set @section = ''; 
set @num = 1; 

SELECT y.* 
FROM 
(
    SELECT 
     x.*, 
     @num := if(@section = sectionid, @num + 1, 1) as row_number, 
     @section := sectionid 
    FROM 
    (
     SELECT 
      cm.id AS cm_id, 
      cm.title AS cmtitle, 
      cm.sectionid, 
      cm.type AS cmtype, 
      cd.id AS cd_id, 
      cd.time, 
      cd.link, 
      cd.title, 
      cd.description 
     FROM c_main AS cm 
     JOIN c_data AS cd ON (cd.sectionid=cm.sectionid) 
     ORDER by cd.sectionid ASC, cm.id ASC 
    ) x 
) y 
WHERE y.row_number <= 20;