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Question

考虑像这样的表

tbl_ranks
--------------------------------
family_id | item_id | view_count 
--------------------------------
1           10        101
1           11        112
1           13        109

2           21        101
2           22        112
2           23        109

3           30        101
3           31        112
3           33        109

4           40        101
4           51        112
4           63        109

5           80        101
5           81        112
5           88        109

我需要生成一个结果集，其中包含按观看次数排序的家庭 ID 子集（例如 1、2、3 和 4）的前两 (2) 行。 我想做类似的事情

select top 2 * from tbl_ranks where family_id = 1 order by view_count
union all
select top 2 * from tbl_ranks where family_id = 2 order by view_count
union all
select top 2 * from tbl_ranks where family_id = 3 order by view_count
union all
select top 2 * from tbl_ranks where family_id = 4 order by view_count

但是，当然，order by 在 union all 上下文中以这种方式无效。有什么建议？我知道我可以运行一组 4 个查询，将结果存储到临时表中，然后选择该临时表的内容作为最终结果，但如果可能的话，我宁愿避免使用临时表。

注意：在实际应用中，每个家庭 id 的记录数是不确定的，并且 view_counts 也不是固定的，如上例所示。

Answer 1

你可以试试这样的

DECLARE @tbl_ranks TABLE(
        family_id INT,
        item_id INT,
        view_count INT
)

INSERT INTO @tbl_ranks SELECT 1,10,101
INSERT INTO @tbl_ranks SELECT 1,11,112
INSERT INTO @tbl_ranks SELECT 1,13,109

INSERT INTO @tbl_ranks SELECT 2,21,101
INSERT INTO @tbl_ranks SELECT 2,22,112
INSERT INTO @tbl_ranks SELECT 2,23,109

INSERT INTO @tbl_ranks SELECT 3,30,101
INSERT INTO @tbl_ranks SELECT 3,31,112
INSERT INTO @tbl_ranks SELECT 3,33,109

INSERT INTO @tbl_ranks SELECT 4,40,101
INSERT INTO @tbl_ranks SELECT 4,51,112
INSERT INTO @tbl_ranks SELECT 4,63,109

INSERT INTO @tbl_ranks SELECT 5,80,101
INSERT INTO @tbl_ranks SELECT 5,81,112
INSERT INTO @tbl_ranks SELECT 5,88,109

SELECT  *
FROm    (
            SELECT  *,
                    ROW_NUMBER() OVER(PARTITION BY family_id ORDER BY view_count DESC) MyOrder
            FROM    @tbl_ranks
        ) MyOrders
WHERE   MyOrder <= 2

Answer 2

如果您使用的是 SQL Server 2005 或更高版本，则可以利用分析功能：

SELECT * FROM (
   SELECT rank() OVER (PARTITION BY family_id ORDER BY view_count) AS RNK, * FROM ...
     )
WHERE RNK <= 2
ORDER BY ...

Answer 3

SELECT  tro.*
FROM    family
CROSS APPLY
        (
        SELECT  TOP 2 *
        FROM    tbl_ranks tr
        WHERE   tr.family_id = family.id
        ORDER BY
                view_count DESC
        ) tro
WHERE   family.id IN (1, 2, 3, 4)

如果您没有实际的 family 表，您可以使用一组联合或递归 CTE 来构造它：

WITH   family AS
       (
       SELECT  1 AS id
       UNION ALL
       SELECT  2 AS id
       UNION ALL
       SELECT  3 AS id
       UNION ALL
       SELECT  4 AS id
       )
SELECT  tro.*
FROM    family
CROSS APPLY
        (
        SELECT  TOP 2 *
        FROM    tbl_ranks tr
        WHERE   tr.family_id = family.id
        ORDER BY
                view_count DESC
        ) tro
WHERE   family.id IN (1, 2, 3, 4)

确保您在tbl_ranks (family_id, viewcount) 上有一个索引。

如果每个家庭有很多等级，这将是有效的，因为如果与 PARTITION BY 一起使用，像 ROW_NUMBER 这样的分析函数将不会使用 TOP 方法。

Answer 4

用途：

SELECT *
  FROM (select *,
               ROW_NUMBER() OVER (PARTITION BY family_id ORDER BY view_count DESC) 'rank'
          from tbl_ranks) x
  WHERE x.rank <= 2
ORDER BY ...

基本原理是分配一个排名，然后根据它进行过滤。

Answer 5

您只需稍微调整建议的 SQL 命令即可使其按您想要的方式工作。要绑定 TOP 和 ORDER BY，您可以将语句放在您从中选择的括号内并给出名称（此处未使用但必需）。

使用来自Adriaan Stander's answer 的 DECLARE 和 INSERT 语句如下

SELECT * FROM (SELECT TOP 2 * FROM @tbl_ranks WHERE family_id = 1 ORDER BY view_count) AS dummy1 UNION ALL
SELECT * FROM (SELECT TOP 2 * FROM @tbl_ranks WHERE family_id = 2 ORDER BY view_count) AS dummy2 UNION ALL
SELECT * FROM (SELECT TOP 2 * FROM @tbl_ranks WHERE family_id = 3 ORDER BY view_count) AS dummy3 UNION ALL
SELECT * FROM (SELECT TOP 2 * FROM @tbl_ranks WHERE family_id = 4 ORDER BY view_count) AS dummy4

给予

family_id   item_id view_count
1   10  101
1   13  109
2   21  101
2   23  109
3   30  101
3   33  109
4   40  101
4   63  109