MySQL，如果在另一个表中找不到匹配的ID

Question

Users
+----+------+
| id | name |
+----+------+
| 1  | John |
+----+------+
| 2  | Jane |
+----+------+

Message
+----+------+--------------+
|u_id| date | msg          |
+----+------+--------------+
| 1  | 0945 | some text1   |
+----+------+--------------+
| 0  | 0950 | some Text2   |
+----+------+--------------+
| 2  | 1045 | some Text3   |
+----+------+--------------+

我希望将“ not_registered_user”文本用作用户名，其中用户ID为0（对于非注册用户，每次为0）。所需的输出将是：

+------+--------------+--------------+
| date | name         | msg          |
+------+--------------+--------------+
| 0945 | John         | some text1   |
+------+--------------+--------------+
| 1045 | Jane         | some text3   |
+------+--------------+--------------+
| 0950 | non_reg_user | some text2   |
+------+--------------+--------------+

请帮助，谢谢。

Answer 1

使用left join和coalesce()

select date,coalesce(name,'non_reg_user') as name,msg
from message a
left join users b on a.u_id=b.id