返回空值 IN 子句 mysql

Question

在数据库中找不到数据时找不到打印。例如在我的数据库中我没有56443，因此它应该打印'not found'

SELECT uid, (CASE WHEN (u.uid = null) THEN 'not found' ELSE 'found' END) as result
FROM (SELECT uid
            FROM users
            WHERE uid IN (1,2,56443,3)) as u;

得到如下结果

+--------+--------+
| uid    | result|
+--------+--------+
| 1      | found | 
| 2      | found |
| 3      | found |
+--------+--------+

我也期待not found 与56443 的行

Answer 1

您需要使用不同的方法。您需要使用 UNION ALL 创建一个包含所有值的内联视图，然后将其与 users 表左连接：

SQL Fiddle

查询 1：

SELECT a.uid, (CASE WHEN (u.uid is null) THEN 'not found' ELSE 'found' END) as     result
FROM (select 1 as UID FROM dual
      UNION ALL
      select 2 as UID FROM dual
      UNION ALL
      select 56443 as UID FROM dual
      UNION ALL
      select 3 as UID FROM dual) as a
LEFT JOIN users u on a.uid = u.uid

[结果]：

|   UID |    result |
|-------|-----------|
|     1 |     found |
|     2 |     found |
|     3 |     found |
| 56443 | not found |

Answer 2

那是因为您正在将一个值与 null aka 进行比较。未知。与空值进行比较时，始终使用 IS 运算符。 CASE WHEN (u.uid is null) THEN 'not found' ELSE 'found' END) 作为结果

试试这个（更新答案）：

SELECT u2.uid, (CASE WHEN (u1.uid is null) THEN 'not found' ELSE 'found' END) 
as     result
FROM users u1
RIGHT JOIN 
(select 1 as uid union all
 select 2 as uid union all
 select 3 as uid union all
 select 56443 as uid
) u2
on u1.uid = u2.uid