向 SQL UNION SELECT 添加附加列答案

【问题标题】：Adding an additional column to SQL UNION SELECT向 SQL UNION SELECT 添加附加列
【发布时间】：2015-09-10 23:50:19
【问题描述】：

假设我有以下映射和规范化的表；

Group                       User                        Contact                     BelongsTo       
+-------+------+------+     +-------+------+------+     +-------+------+------+     +-------+------+
|    gID|  name|  col3|     |    uID| fname| sname|     |    cID|  name|  col3|     |  accID|   gID|
+-------+------+------+     +-------+------+------+     +-------+------+------+     +-------+------+
|1      |ABC   |?     |     |1      |JJ    |BB    |     |4      |ABCD  |?     |     |1      |2     |
+-------+------+------+     +-------+------+------+     +-------+------+------+     +-------+------+
|2      |BCD   |?     |     |2      |CC    |LL    |     |5      |BCDE  |?     |     |3      |2     |
+-------+------+------+     +-------+------+------+     +-------+------+------+     +-------+------+
|3      |DEF   |?     |     |3      |RR    |NN    |     |6      |CDEF  |?     |     |5      |3     |
+-------+------+------+     +-------+------+------+     +-------+------+------+     +-------+------+

使用 EERM，User 和 Contact 是“Account”超类的子类。（未显示）一个帐户可以属于许多组，因此“BelongsTo”表记录了帐户和组成员之间的 M:N 关系。

我想要一条 SQL 语句，它允许我查询在组匹配条件中具有成员资格的所有用户和联系人，如下所示：

SELECT
    tc."cID" AS "accID",
    tc."name" AS "accName",
    tbt."gID"
FROM "tblContact" tc
    INNER JOIN "tblBelongsTo" tbt
        ON tbt."accID" = tc."cID"
UNION SELECT
    tu."uID" AS "accID",
    CONCAT (tu."fname", ' ', tu."sname") AS "accName",
    tbt."gID"
FROM "tblUser" tu
    INNER JOIN "tblBelongsTo" tbt
        ON tbt."accID" = tu."uID"
ORDER BY "accID" ASC;

以上工作，我在查询中结合了 UNION SELECT，因为当我将名字和姓氏连接在一起时，列数匹配任一侧。导致全局“account_name”和“account_id”列。

我的问题是：我将如何添加一个额外的列以便我可以看到组名是什么？

来自这个：

Result
+-------+-------+------+
|  accID|accName|   gID|
+-------+-------+------+
|1      |JJBB   |2     |
+-------+-------+------+
|3      |RRNN   |2     |
+-------+-------+------+
|5      |BCDE   |3     |
+-------+-------+------+

到这里：

Result (2)
+-------+-------+------+------+
|  accID|accName|   gID|  name|
+-------+-------+------+------+
|1      |JJBB   |2     |   BCD|
+-------+-------+------+------+
|3      |RRNN   |2     |   BCD|
+-------+-------+------+------+
|5      |BCDE   |3     |   DEF|
+-------+-------+------+------+

似乎我尝试过的所有操作都会导致 UNION SELECT 中断（因为列不匹配）。同样，我在组合子查询方面运气不佳。我可能遗漏了一些非常明显的东西......

提前致谢。

【问题讨论】：

标签： sql select inner-join union

【解决方案1】：

您可以将join 发送到group 表以获取name。

select x.*, g.name
from group g join
(
SELECT
tc."cID" AS "accID",
tc."name" AS "accName",
tbt."gID"
FROM "tblContact" tc
INNER JOIN "tblBelongsTo" tbt
ON tbt."accID" = tc."cID"
UNION 
SELECT
tu."uID" AS "accID",
CONCAT (tu."fname", ' ', tu."sname") AS "accName",
tbt."gID"
FROM "tblUser" tu
INNER JOIN "tblBelongsTo" tbt
ON tbt."accID" = tu."uID"
) x on x.gid = g.gid
order by accid;

【讨论】：

完美！谢谢你。 :)