SQL If field is '*' then select from another table答案

【问题标题】：SQL If field is '*' then select from another tableSQL If field is '*' then select from another table
【发布时间】：2020-11-10 14:54:08
【问题描述】：

我有一张名为“订单”的表格。我的表中有一个名称为 adressFinder('*', '%', '+') 的 1 个字符的字段。如果有“+”，我想从“客户”表中选择“地址”字段。如果有“%”，我想从“customerTwo”表中选择“adress”，如果有“+”，则从“customerThree”表中选择“adress”。

所有表格都可以通过“customerID”字段连接。

我尝试了类似的方法但不起作用（语法问题）：

Select order.orderId, order.customerID,
CASE WHEN order.adressFinder = '*' THEN (SELECT customer.adress FROM customer INNER JOIN order on (customer.customerID = order.customerID)) 
     WHEN order.adressFinder = '%' THEN (SELECT customer.adress FROM customerTwo INNER JOIN order on (customerTwo.customerID = order.customerID)) 
     WHEN order.adressFinder = '+' THEN (SELECT customer.adress FROM customerThree INNER JOIN order on (customerThree.customerID = order.customerID))
END
FROM order
INNER JOIN customer on (order.customerID = customer.customerID)
INNER JOIN customerTwo on (order.customerID = customerTwo.customerID) 
INNER JOIN customerThree on (order.customerID = customerThree.customerID)

有人可以帮我解决我的问题吗？谢谢！

【问题讨论】：

样本数据和期望的结果会有很大帮助。

标签： mysql sql database if-statement

【解决方案1】：

您可以使用 LEFT JOIN 来做到这一点。假设您只想显示一个地址，您可以使用 COALESCE - 它会将第一个非空值显示为地址。

例如：

SELECT
o.orderId AS orderId, 
o.customerID AS customerID,
COALESCE(c.adress, c2.adress, c3.adress) AS Address
FROM order o
LEFT JOIN customer c ON c.customerID = o.customerID AND o.adressFinder = '*'
LEFT JOIN customerTwo c2 ON c2.customerID = o.customerID AND o.adressFinder = '%'
LEFT JOIN customerThree c3 ON c3.customerID = o.customerID AND o.adressFinder = '+'

【讨论】：

这对我帮助很大，但是有没有办法做到这一点，如果有另一个字段包含字符（*、%、+）并且你必须再做一次？

【解决方案2】：

我不明白您为什么要使用子查询。这不是你想要的吗？

SELECT o.orderId, o.customerID,
       COALESCE(c.address, c2.address, c3.address)
FROM order o LEFT JOIN
     customer c
     ON o.customerID = c.customerID AND
        o.adressFinder = '*' LEFT JOIN
     customerTwo c2
     ON o.customerID = c2.customerID AND
        o.adressFinder = '%' LEFT JOIN
     customerThree c3
     ON o.customerID = c3.customerID AND
        o.adressFinder = '+';

【讨论】：