sql - SQL：级联 UNION 和 JOIN

Question

我在两个表之间有一个联合操作

SELECT ID_1,
       name_1,
       surname_1,
  FROM T_ONE
 UNION
SELECT ID_2,
       name_2,
       surname_2
  FROM TABLE_2

我想将此UNION操作的结果与另一个表甚至全部加入TABLE_1。

如何处理UNION.

例如在上一个之后UNION：

RIGHT JOIN TABLE_3
        ON TABLE_3.ID_3 = XXXXXXXXXXXXXXXXXXXX.ID_2

我真的不知道我需要放什么来代替 XXXXXXXXXXXXXXXX 来处理由UNION.

Answer 1

在此处使用“foo”之类的派生表，然后根据需要再次加入：

SELECT
    *
FROM
    TABLE_3
    LEFT JOIN
    (
    SELECT ID_1, name_1, surname_1, FROM T_ONE
    UNION --ALL would be more efficient if results do not overlap, as van's comment said
    SELECT ID_2, name_2, surname_2 FROM TABLE_2
    ) foo  ON TABLE_3.ID_3 = foo.ID_1

PS。使用左连接：比右连接更容易混淆。

Answer 2

您需要在两个 SELECT 中提供连接：

SELECT ID_1, name_1, surname_1, FROM T_ONE
RIGHT JOIN TABLE_3 ON TABLE_3.ID_3 = T_ONE.ID_1

UNION

SELECT ID_2, name_2, surname_2 FROM TABLE_2
RIGHT JOIN TABLE_3 ON TABLE_3.ID_3 = TABLE_2.ID_2

或类似的东西。不要忘记 UNION 会消除重复项，因此如果您希望包含重复项，请使用 UNION ALL

Answer 3

SELECT ID_1, name_1, surname_1, FROM T_ONE

from
(SELECT ID_1, name_1, surname_1, FROM T_ONE
UNION 
SELECT ID_2, name_2, surname_2 FROM TABLE_2 ) foo

left join TABLE_3 

ON TABLE_3.ID_3 =foo.ID_2

Answer 4

为那些问过的人扩展上面的 gbn 答案。以下是如何将联合作为左连接的第一部分。

SELECT table3.phone, foo.fname, foo.lname, foo.email
FROM
(SELECT table1.fname,table1.lname,table1.email
    FROM table1
UNION
SELECT table2.fname,table2.lname,table2.email
    FROM table2
) foo
LEFT JOIN table3
ON foo.lname = table3.lname