SQL Server +为表中每个masterID只选择两条记录

Question

我得到一个子表，它包含通过MasterID列链接到主表的1到n条记录。

如何从子表中选择每个MasterID的前5个记录？

Answer 1

使用SQL Server CTE和ROW_NUMBER你可以尝试使用

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子表拉动TOP 5。在未经测试的伪代码：

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)