SQL左側的接合部3個表

Question

我有1個表：

Contacts (ID integer, Name text, ATT_ID integer) 

我的表是充滿了這些值：

(1, 'Alice', 1) 
(2, 'Bob', 1) 
(3, 'Carol', 1) 
(4, 'Dave', 4) 
(5, 'Eve', 4) 
(6, 'Frank', 6) 

的目標是參加這些聯繫人ID與配對ATT_ID。

這是我當前SQL代碼：

SELECT t1.ID as ID, t1.Name , tt.Name , tt2.Name 
FROM Contacts as t1 

LEFT JOIN (
SELECT MIN(t2.ID), t2.Name, t2.ATT_ID FROM Contacts as t2 
WHERE t2.ID <> t2.ATT_ID) 
AS tt ON t1.ID = tt.ATT_ID 

LEFT JOIN (
SELECT MAX(t3.ID), t3.Name, t3.ATT_ID FROM Contacts as t3 
WHERE t3.ID <> t3.ATT_ID) 
AS tt2 ON t1.ID = tt2.ATT_ID 

WHERE t1.ID = t1.ATT_ID; 

，我的結果是：

Alice | Bob | null 
Dave | null | Eve 
Frank | null | null 

但期望的結果應該是這樣的：

Alice | Bob | Carol 
Dave | Eve | null 
Frank | null | null 

我怎麼能完成這個？

Answer 1

假設你的RDBMS支持窗口功能...

這可以通過產生的行號爲每個子組僅顯示的最小行數來實現。

這也假設你在每組中不超過3個。

以下是特定於ORACLE，但應該與MS SQL和Postgresql和DB2一起工作。由於不支持窗口功能，它不適用於MySQL。

請注意，構建的塊只能在ORACLE中使用。

With contacts (ID, Name, ATT_ID) as ( 
Select 1, 'Alice', 1 from dual union all 
Select 2, 'Bob', 1 from dual union all 
Select 3, 'Carol', 1 from dual union all 
Select 4, 'Dave', 4 from dual union all 
Select 5, 'Eve', 4 from dual union all 
Select 6, 'Frank', 6 from dual) 

--FROM HERE ON should work if window functions supported. 

Select * from (
Select T1.Name N1, T2.Name N2, T3.Name N3, Row_Number() Over (partition by T1.ATT_ID order by T1.ID) rn 
FROM Contacts T1 
LEFT JOIN Contacts T2 
on T1.ATT_ID = T2.ATT_ID 
and T1.ID < T2.ID 
LEFT JOIN contacts T3 
on T2.ATT_ID = T3.ATT_ID 
and T2.ID < T3.ID 
and T1.ID < T3.ID) B 
WHERE RN =1