Question

我有一个带有Owner_ID字段的Vehicle表。此Owner_ID将对应于Customer表中的ID字段或Business表中的ID字段。我希望根据ID所属的表返回不同的值。

我有这样的声明：

SELECT v.Make, v.Model
FROM Vehicle v
LEFT JOIN Customer c ON c.ID = v.Owner_ID
LEFT JOIN Business b ON b.ID = v.Owner_ID

所以说如果Owner_ID = Customer.ID，我想返回客户姓氏，如果Owner_ID = Business_ID，则返回公司名称。我知道我可以使用CASE语句：

SELECT v.Make, v.Model,
    CASE WHEN c.ID IS NOT NULL THEN c.Surname
        WHEN b.ID IS NOT NULL THEN b.Name

但是有没有办法从一个case语句中获取多个值？类似的东西:(我知道这样做完全是错的。）

SELECT v.Make, v.Model,
    CASE WHEN c.ID IS NOT NULL THEN
            SELECT c.Surname, c.Date_Of_Birth
        WHEN b.ID IS NOT NULL THEN
            SELECT b.Name, b.Founded

或者我必须这样做：

SELECT v.Make, v.Model,
    CASE WHEN c.ID IS NOT NULL THEN c.Surname
        WHEN b.ID IS NOT NULL THEN b.Name
    END AS "Name",
    CASE WHEN c.ID IS NOT NULL THEN c.Date_Of_Birth
        WHEN b.ID IS NOT NULL THEN b.Founded
    END AS "DOB/Founded"

Answer 1

我认为需要明确查询应该返回哪些列，这不能根据查询而改变。因此，每列必须分解条件。

IF怎么样？在这种情况下可能更容易阅读。

SELECT v.Make, v.Model,
    IF(c.ID IS NOT NULL, c.Surname,       IF(b.ID IS NOT NULL, b.Name,    NULL)) as `Name`
    IF(c.ID IS NOT NULL, c.Date_Of_Birth, IF(b.ID IS NOT NULL, b.Founded, NULL)) as `DOB_Founded`

Answer 2

没有办法让case表达式的分支跨越多列。

在您的示例中，您可以执行

SELECT v.Make,
       v.Model,
       MAX(COALESCE(c.Surname, b.name)) as name
FROM   Vehicle v
       LEFT JOIN Customer c
         ON c.ID = v.Owner_ID
       LEFT JOIN Business b
         ON b.ID = v.Owner_ID
GROUP  BY v.Owner_ID

Answer 3

不确定，但你可以试试这个

SELECT v.Make, v.Model
FROM Vehicle v
LEFT JOIN Customer c ON c.ID = v.Owner_ID
LEFT JOIN Business b ON b.ID = v.Owner_ID 
WHERE c.Surname IS NOT NULL 
OR b.name IS NOT NULL

通过CASE语句选择多个值？

3 个答案: