Question

是否可以LEFT JOIN两个具有相同性质的列，即将t2.Name和t3.Name加入到T-SQL语句中的单个列（dataproperty）Name中？

SELECT t1.ID,
       t1.RowID,
       t2.name, 
       t3.Name 
  FROM MasterTable T1 
       LEFT OUTER JOIN RightTable1 T2 
            ON T1.RowID = T2.ID 
       LEFT OUTER JOIN RightTable2 T3 
            ON T1.RowID = T3.ID 
 WHERE t1.ID= " & ID & ";

...或者我是否必须将表格加载到单独的数据集中并以编程方式（对于每个...下一个）将它们连接到新数据集中？

此致

Libor的

Answer 1

这取决于你所说的帽子＆＃34;加入＆＃34; - 这里有几种可能性：

如果一个为空而另一个为空，则使用ISNULL：

SELECT DISTINCT 
       t1.ID,
       t1.RowID,
       ISNULL(t2.name, t3.Name) Name
FROM MasterTable T1 
LEFT OUTER JOIN RightTable1 T2 on T1.RowID = T2.ID 
LEFT OUTER JOIN RightTable2 T3 on T1.RowID = T3.ID

如果你想连接它们：

SELECT DISTINCT 
       t1.ID,
       t1.RowID,
       t2.name + t3.Name Name
FROM MasterTable T1 
LEFT OUTER JOIN RightTable1 T2 on T1.RowID = T2.ID 
LEFT OUTER JOIN RightTable2 T3 on T1.RowID = T3.ID

如果你想要两行（一个UNION）：

SELECT DISTINCT 
       t1.ID,
       t1.RowID,
       t2.Name
FROM MasterTable T1 
LEFT OUTER JOIN RightTable1 T2 on T1.RowID = T2.ID 
UNION ALL
SELECT DISTINCT 
       t1.ID,
       t1.RowID,
       t3.Name
FROM MasterTable T1 
LEFT OUTER JOIN RightTable2 T3 on T1.RowID = T3.ID

请注意，DISTINCT是必要的，因为除非您可以关联T2和T3，否则您基本上在T2和T3之间进行交叉连接（笛卡尔积），因此您需要消除所创建的重复项。

Answer 2

SELECT t1.ID,
       t1.RowID,
       t2.name + t3.Name AS JoinedName
  FROM MasterTable T1 
       LEFT OUTER JOIN RightTable1 T2 
            ON T1.RowID = T2.ID 
       LEFT OUTER JOIN RightTable2 T3 
            ON T1.RowID = T3.ID 
 WHERE t1.ID= " & ID & ";

LEFT JOIN将不明确的列放入SQL语句的一列中？

2 个答案: