我一直在堆栈中搜索类似的问题,但是找不到解决这个问题的方法,或者我听不懂。我有3张桌子。
Users
+---------+------+--------------+
| id_user | name | age |
+---------+------+--------------+
| user1 | John | 51 |
+---------+------+--------------+
| user2 | Jane | 65 |
+---------+------+--------------+
| user3 | Katie| 51 |
+---------+------+--------------+
| user4 | Marck| 65 |
+---------+------+--------------+
City1
+---------+------+--------------+
| id_user | time | street |
+---------+------+--------------+
| user1 | 8 | 111111111111 |
+---------+------+--------------+
| user2 | 5 | 222222222222 |
+---------+------+--------------+
City2
+---------+------+--------------+
| user_id | time | street |
+---------+------+--------------+
| user1 | 6 | 111111111111 |
+---------+------+--------------+
| user4 | 7 | 222222222222 |
+---------+------+--------------+
我正在尝试进行查询,以了解City1和City2表中是否存在特定的id_user。我真的不知道如何构造它。例如,完成id_user = user1,我想收到诸如city1 = true,city2 = true之类的信息。 (在两个表中,user1存在);或完成id_user = 2,获得city1 = true,city2 = false。
有关如何执行此操作的任何想法?
如果没有在每个表中重复用户ID,则可以使用left outer join
:
select u.*,
(c1.id_user is not null) as InCity1,
(c2.id_user is not null) as InCity2
from users u left outer join
city1 c1
on u.id_user = c1.id_user left outer join
city2 c2
on u.id_user = c2.id_user;
如果您只想查看一位用户的信息,则可以添加where
子句。