SQL JOIN表與子表的

Question

假設下面的示例表：

+-----+-----+-----+ 
| ID1 | ID2 | VAL | 
+-----+-----+-----+ 
| A | X | 1 | 
| A | Y | 2 | 
| A | Z | 3 | 
| B | X | 4 | 
| B | Z | 5 | 
| C | Y | 6 | 
+-----+-----+-----+ 

說我很感興趣，ID1 = B並希望它和別人比較。期望的輸出將是：

+---------+---------+---------+---------+---------+---------+---------+ 
| REF.ID1 | OTH.ID1 | REF.ID2 | REF.ID2 | REF.VAL | OTH.VAL | DIFF | 
+---------+---------+---------+---------+---------+---------+---------+ 
| B  | A  | X  | X  | 4  | 1  | 3  | 
| B  | A  | NULL | Y  | NULL | 2  | -2  | 
| B  | A  | Z  | Z  | 5  | 3  | 2  | 
| B  | B  | X  | X  | 4  | 4  | 0  | 
| B  | B  | Z  | Z  | 5  | 5  | 0  | 
| B  | C  | NULL | Y  | NULL | 6  | -6  | 
+---------+---------+---------+---------+---------+---------+---------+ 

謝謝。

Answer 1

你需要的是一個交叉聯接：

SELECT Ref.Id, Oth.id, ...... 
FROM dbo.tbl AS Ref 
CROSS JOIN dbo.tbl AS Oth 
WHERE Ref.Id = 'B'; 

欲瞭解更多關於加入這裏看看我的博客系列：http://sqlity.net/en/1146/a-join-a-day-introduction/

交叉聯接這裏描述：http://sqlity.net/en/1183/a-join-a-day-the-cross-join/

Answer 2

這應該做：

SELECT A.ID1 [REF.ID1], 
     A.ID1 [OTH.ID1], 
     CASE WHEN A.ID2 = B.ID2 THEN A.ID2 ELSE NULL END [REF.ID2], 
     B.ID2 [OTH.ID2], 
     CASE WHEN A.ID2 = B.ID2 THEN A.VAL ELSE NULL END [REF.VAL], 
     B.VAL [OTH.VAL], 
     CASE WHEN A.ID2 = B.ID2 THEN A.VAL ELSE 0 END - B.VAL [DIFF] 
FROM ( SELECT * 
     FROM YourTable 
     WHERE ID1 = 'B') A 
CROSS JOIN YourTable B