从两个连接表中仅选择一个结果字段答案

【问题标题】：Select only one result field from two joined tables从两个连接表中仅选择一个结果字段
【发布时间】：2020-05-28 09:58:12
【问题描述】：

我试图从两个表中只选择一个字段。而不是c.cust_name 和s.supp_name 只有一个字段名称与customer 或supplier 表的竞争。

SELECT o.id, o.order_nr, o.cust_suppl, c.cust_name, s.supp_name
FROM   Orders AS o LEFT OUTER JOIN
       customer AS c ON o.id = c.id LEFT OUTER JOIN
       supplier AS s ON o.id = s.id

当前输出：

id    order_nr   cust_suppl   cust_name   supp_name
1       1100       c            Cust_A      NULL    
2       1101       c            Cust_B      NULL
3       2100       s            NULL        Suppl_A
4       2101       s            NULL        Suppl_B

【问题讨论】：

您想如何进行选择？基于cust_suppl?

标签： sql join

【解决方案1】：

你可以解码：

SELECT o.id, o.order_nr, o.cust_suppl
       , decode( c.cust_name, null, s.supp_name, c.cust_name )
FROM   Orders AS o LEFT OUTER JOIN
       customer AS c ON o.id = c.id LEFT OUTER JOIN
       supplier AS s ON o.id = s.id

或使用案例：

SELECT o.id, o.order_nr, o.cust_suppl
       , case when c.cust_name is null then s.supp_name else c.cust_name end
FROM   Orders AS o LEFT OUTER JOIN
       customer AS c ON o.id = c.id LEFT OUTER JOIN
       supplier AS s ON o.id = s.id

【讨论】：