每個類別的MySQL限制結果

Question

c_data中的每個項目都在一個類別/部分中。我想限制每個類別顯示多少項目，而不是限制檢索項目的總數。很顯然，如果我在查詢中添加「限制20」之類的內容，它只會總共獲取20個結果，而不是每個類別的20個結果。

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 

該分類的字段是「sectionid」。

Answer 1

MySQL沒有任何等級的功能，但你可以使用一個變量來創建一個僞行號。

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

這個previous post的答案應該可以幫助你解決這個問題。

編輯：

應該用row numbers工作。

我還沒有嘗試過，但是這應該工作：

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;