SQL - 检查多个表中是否存在记录

Question

我一直在堆栈中搜索类似的问题,但我找不到一个帮助我解决这个问题,或者我无法理解它.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建它.例如,完成id_user = user1,我想收到类似city1 = true,city2 = true的内容.(在两个表中,user1都存在); 或者完成id_user = 2,收到city1 = true,city2 = false.

有关如何做到这一点的任何想法？

非常感谢

Answer 1

如果每个表中没有重复用户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;

where如果要仅查看一个用户的信息,可以添加子句.