多個完整外部聯接

Question

我想在另一個表上使用FULL OUTER JOIN的結果作爲FULL OUTER JOIN的表。我應該用什麼語法？

對於例如：T1，T2，T3是我的表，列id，名稱。我需要這樣的東西：

T1 FULL OUTER JOIN T2 on T1.id = T2.id ==> Let this be named X 

X FULL OUTER JOIN T3 on X.id = t3.id 

我希望這可以實現，這樣在決賽條款，我希望T3.id要匹配T1.id或T2.id。任何替代方法都可以。

Answer 1

SELECT COALESCE(X.id,t3.id) AS id, *-- specific columns here instead of the * 
FROM 
    (
     SELECT COALESCE(t1.id,t2.id) AS id, * -- specific columns here instead of the * 
     FROM T1 FULL OUTER JOIN T2 on T1.id = T2.id 
    ) AS X 
    FULL OUTER JOIN T3 on X.id = t3.id 

Answer 2

你可以像你的建議，使用IN（）

FROM T1 
FULL OUTER JOIN T2 
ON(T1.id = T2.id) 
FULL OUTER JOIN T3 
ON(T3.ID IN(T2.id,T1.id)) 

或者我認爲你可以使用UNION做（取決於你的需要）：

SELECT * FROM 
    (SELECT name,id from T1 
    UNION 
    SELECT name,id from T2) x 
FULL OUTER JOIN T3 
    ON(t3.id = x.id) 

Answer 3

通常，完全外連接的鏈條行爲並不像預期的那樣。一個替代品使用left join。當表格中包含所有您需要的ID時，此功能效果最佳。但是你也可以構造：

from (select id from t1 union 
     select id from t2 union 
     select id from t3 
    ) ids left join 
    t1 
    on ids.id = t1.id left join 
    t2 
    on ids.id = t2.id left join 
    t3 
    on ids.id = t3.id 

請注意，第一個子查詢通常可以用表替換。如果你有這樣一個表，你可以選擇where子句中的匹配行：

from ids left join 
    t1 
    on ids.id = t1.id left join 
    t2 
    on ids.id = t2.id left join 
    t3 
    on ids.id = t3.id 
where t1.id is not null or t2.id is not null or t3.id is not null