Question

我知道在SQL语句结尾处有一个LIMIT，如下所示：

 SELECT * FROM books WHERE 1 LIMIT 3

它最多会显示表books中的3个项目。

现在假设我们有一个字段category并按其过滤：

 SELECT * FROM books WHERE category IN (1, 3, 5) LIMIT 3

我将获得具有值为1或3或5的类别的行，并且再次将行数限制为3。

如何为每个类别限制3？（基本思想概要）

 SELECT * FROM books WHERE category IN (1, 3, 5) LIMITS (3, 3, 3)

Answer 1

您可以使用UNION：

执行此操作

(SELECT * FROM books WHERE category = 1 LIMIT 3)
UNION 
(SELECT * FROM books WHERE category = 3 LIMIT 3)
UNION 
(SELECT * FROM books WHERE category = 5 LIMIT 3)

http://dev.mysql.com/doc/refman/5.7/en/union.html

Answer 2

与MySQL user defined variables一起，你可以做到这一点：

SELECT 
t.*
FROM 
(
    SELECT 
    *,
    IF( @sameCategory = category , @rn := @rn + 1,
         IF(@sameCategory := category, @rn := 1, @rn := 1)
    ) AS rowNumber
    FROM books
    CROSS JOIN (SELECT @sameCategory := 0, @rn := 1) var
    ORDER BY category 
) AS t
WHERE t.rowNumber <= 3
ORDER BY t.category

Answer 3

您不会在单个查询中使用LIMIT。一种方法使用变量：

select b.*
from (select b.*,
             (@rn := if(@c = category, @rn + 1,
                        if(@c := category, 1, 1)
                       )
             ) as rn
      from books b cross join
           (select @c := -1, @rn := 0) params
      where category in (1, 2, 3)
      order by category
     ) b
where rn <= 3;

如何在SQL Select中创建部分LIMIT

3 个答案: