SQL Server +仅为表中的每个masterID选择两个记录

Question

我有一个子表，其中包含1到n条通过MasterID列链接到主表的记录。

如何从子表中为每个MasterID仅选择前5条记录？

Answer 1

使用Sql Server CTE和ROW_NUMBER，您可以尝试使用

DECLARE @ParentTable TABLE(
        ID INT
)

INSERT INTO @ParentTable SELECT 1
INSERT INTO @ParentTable SELECT 2
INSERT INTO @ParentTable SELECT 3

DECLARE @ChildTable TABLE(
        ID INT,
        ParentID INT
)

INSERT INTO @ChildTable SELECT 1, 1
INSERT INTO @ChildTable SELECT 2, 1
INSERT INTO @ChildTable SELECT 3, 1
INSERT INTO @ChildTable SELECT 4, 1
INSERT INTO @ChildTable SELECT 5, 1
INSERT INTO @ChildTable SELECT 6, 1
INSERT INTO @ChildTable SELECT 7, 1

INSERT INTO @ChildTable SELECT 8, 2
INSERT INTO @ChildTable SELECT 9, 2

INSERT INTO @ChildTable SELECT 10, 3
INSERT INTO @ChildTable SELECT 11, 3

;WITH RowNums AS(
        SELECT  pt.ID ParentID,
                ct.ID ChildID,
                ROW_NUMBER() OVER (PARTITION BY pt.ID ORDER BY ct.ID) RowNum
        FROM    @ParentTable pt INNER JOIN
                @ChildTable ct ON pt.ID = ct.ParentID
)
SELECT  ParentID,
        ChildID
FROM    RowNums
WHERE   RowNum <= 5

Answer 2

尝试常规连接，其中约束是一个子查询，该子查询从子表中拉出TOP 5。 在未经测试的伪代码中：

SELECT A.MasterID, B.*
FROM MasterTable A 
JOIN ChildTable B
ON A.MasterID = B.MasterID 
AND B.ChildID IN (SELECT Top 5 ChildID FROM ChildTable 
    WHERE MasterID = A.MasterID ORDER BY Whatever)