Question

我有两张桌子：[TEXT]，[CATEGORY] 每个文本项都有一个类别以下sql语句检索具有“art”类别

的文本项

SELECT * from [TEXT], [CATEGORY] WHERE [CATEGORY].[id]=[TEXT].[CATEGORY_id] AND
[CATEGORY].[name] IN ("Art") LIMIT 10

我想要做的是从每个类别中检索10个文本项目。

10来自艺术，10来自哲学，10来自科学......

Answer 1

假设你确实意味着MySQL，最快的解决方案涉及变量：

select t.*
from (SELECT *,
             @rn := if(@cname = c.name, @rn + 1, 1) as rn
             @cname := c.name
      from `TEXT` t join
           CATEGORY c
           on c.id = t.CATEGORY_id cross join
           (select @cname := '', @rn := 0) const
      order by c.name
    ) t
where rn <= 10;

我特别不喜欢这个解决方案，因为MySQL不保证在@rn的分配之前发生@cname的分配。但是，它确实在实践中发挥作用。

编辑：

以上在MySQL中有效。在SQLite中，您需要使用查询进行计数：

select t.*
from (SELECT *,
             (select count(*) from Text t2 where t2.Category_id = t.Category_Id and t2.id <= t.id
             ) as seqnum
      from TEXT t join
           CATEGORY c
           on c.id = t.CATEGORY_id 
    ) t
where seqnum <= 10;

这实际上是标准的SQL，但通常性能不太好，所以其他选择是首选。

注意：这假设Text中有一些列标识每一行。我称之为id。

Answer 2

试试这个：

SELECT
    CATEGORY.NAME,
    @curRank := @curRank + 1 AS rank,
    TEXT.NAME    
FROM TEXT 
JOIN ( select NAME.ID, @curRank := 0 from CATEGORY) CATEGORY
       ON CATEGORY.ID = TEXT.ID
) CATEGORY
  ON CATEGORY.ID = TEXT.ID
WHERE rank <= 10
ORDER BY 
    CATEGORY.NAME,
    rank

Answer 3

    SELECT * FROM [CATEGORY] 
    JOIN (SELECT [TEXT].[CATEGORY_id], 
    IF( @previous = [TEXT].[CATEGORY_id],
          @temp := @temp + 1, @temp := 1) cnt, 
          @previous := [TEXT].[CATEGORY_id] 
    FROM  [TEXT] ORDER BY [TEXT].[CATEGORY_id] ) t 
    WHERE [CATEGORY].[id]= t.[CATEGORY_id] AND
    [CATEGORY].[NAME] IN ("Art") and cnt <= 10

INNER JOIN，第二个表格中的行数有限

3 个答案: