如何通过 JOIN 组合（合并）相似的列以删除 NULL

问题简介：

我有一个名为“客户”的超类表和两个从客户继承的名为“个人”和“公司”的子表。因此，客户实体与“人”或“公司”具有一对一的关系。

（客户只能是“个人”或“公司”，但不能同时是两者）

如下所示：

Customer                    Person                      Company                
+-------+------+------+     +-------+------+------+     +-------+------+------+
|    cID|  col2|  col3|     |    cID| fname| sname|     |    cID|  name|  col3|
+-------+------+------+     +-------+------+------+     +-------+------+------+
|1      |?     |?     |     |1      |JJ    |AZ    |     |4      |ABCD  |?     |
+-------+------+------+     +-------+------+------+     +-------+------+------+
|2      |?     |?     |     |2      |CC    |LL    |     |5      |BCDE  |?     |
+-------+------+------+     +-------+------+------+     +-------+------+------+
|3      |?     |?     |     |3      |OO    |BB    |     |6      |CDEF  |?     |
+-------+------+------+     +-------+------+------+     +-------+------+------+
|4      |?     |?     |     |7      |JK    |NN    |     |8      |DEFG  |?     |
+-------+------+------+     +-------+------+------+     +-------+------+------+
|5      |?     |?     |     |9      |RR    |LW    |     |...    |EFGH  |?     |
+-------+------+------+     +-------+------+------+     +-------+------+------+
|6      |?     |?     |     |10     |GN    |QN    |     |...    |FGHI  |?     |
+-------+------+------+     +-------+------+------+     +-------+------+------+
|7      |?     |?     |     |...    |XC    |YU    |     |...    |GHIJ  |?     |
+-------+------+------+     +-------+------+------+     +-------+------+------+
|8      |?     |?     |
+-------+------+------+
|9      |?     |?     |
+-------+------+------+
|10     |?     |?     |
+-------+------+------+
|...    |?     |?     |
+-------+------+------+

• Person 表将客户的全名存储在单独的名字和姓氏属性中。
• 公司表将公司名称完全存储在一个字段中。

意图与尝试：

我想做的是查询数据库，以便我可以从客户表中选择 ID，加入个人和公司以检索名称属性。

以下是我尝试过的：

SELECT      tc."cust_id",
            CONCAT(tp."forename", ' ', tp."surname") AS "name",
            tcp."name"
FROM        "tbl_customer" AS tc
            LEFT JOIN "tbl_person" AS tp
                ON tc."cust_id" = tp."cust_id"
            LEFT JOIN "tbl_company" AS tcp
                ON tc."cust_id" = tcp."cust_id"

执行上面的 SQL 会得到以下结果，右边是我想要实现的结果：

Result                      Result          
+-------+------+------+     +-------+------+
|    cID|  name|  name|     |    cID|  name|
+-------+------+------+     +-------+------+
|1      |JJAZ  |null  |     |1      |JJAZ  |
+-------+------+------+     +-------+------+
|2      |CCLL  |null  |     |2      |CCLL  |
+-------+------+------+     +-------+------+
|3      |OOBB  |null  |     |3      |OOBB  |
+-------+------+------+     +-------+------+
|4      |null  |ABCD  |     |4      |ABCD  |
+-------+------+------+     +-------+------+
|5      |null  |BCDE  |     |5      |BCDE  |
+-------+------+------+     +-------+------+
|6      |null  |CDEF  |     |6      |CDEF  |
+-------+------+------+     +-------+------+
|7      |JKNN  |null  |     |7      |JKNN  |
+-------+------+------+     +-------+------+
|8      |null  |DEFG  |     |8      |DEFG  |
+-------+------+------+     +-------+------+
|9      |RRLW  |null  |     |9      |RRLW  |
+-------+------+------+     +-------+------+
|10     |GNQN  |null  |     |10     |GNQN  |
+-------+------+------+     +-------+------+
|...    |?     |?     |     |...    |?     |
+-------+------+------+     +-------+------+

描述：

如前所述，我试图将个人和公司的名称合并在同一列下。两个表上的标准 JOIN 将不起作用，因为它将返回 NULL 结果。由于 LEFT JOIN 的性质，预计会出现 NULL 值。这可以通过 SQL UNION 非常简单地解决，我知道解决方案，但是我正在寻找通过 JOIN 来替代 UNION 运算符的方法。

我是否可以执行 JOIN、分组/合并名称列？或者类似的东西？但不是一定要用SQL UNION吗？

Update:

两个答案均来自胡安·卡洛斯·奥罗佩萨 and Becuzz都同样可以接受。

COALESCE 听起来它会完全按照您的要求进行。 COALESCE 是一个返回其参数的第一个非 NULL 值的函数。

SELECT      tc."cid",
            COALESCE(tp.firstName||' '||tp.lastName, tcp.name) as "name"
FROM        "customers" AS tc
            LEFT JOIN "person" AS tp
                ON tc."cid" = tp."cid"
            LEFT JOIN "company" AS tcp
                ON tc."cid" = tcp."cid"

SQL小提琴 http://sqlfiddle.com/#!15/c3018/8