postgresql怎么三张表连接查询？

Question

现在我有三张表，表A 、表B、表C，
这三张表都有一个公共字段user_id,且这三个表的user_id都是唯一的，
我要怎么根据某一个user_id把这三张表连接起来查？

Answer 1

postgres=# select * from a;
 user_id | name
---------+------
       1 | a
       2 | aa
       3 | aaa
(3 rows)

postgres=# select * from b;
 user_id | name
---------+------
       4 | b
       5 | bb
       6 | bbb
(3 rows)

postgres=# select * from c;
 user_id | name
---------+------
       7 | c
       8 | cc
       9 | ccc
(3 rows)

postgres=# select * from a union all select * from b union all select * from c order by user_id;
 user_id | name
---------+------
       1 | a
       2 | aa
       3 | aaa
       4 | b
       5 | bb
       6 | bbb
       7 | c
       8 | cc
       9 | ccc
(9 rows)