Clever Geek Handbook

How to select specific data from 3 tables? [PostgreSQL 9.1]

# select * from users ;
 user_id | login  |          email          | password | firstname | lastname 
---------+--------+-------------------------+----------+-----------+----------
       1 | katie  | katie.s@ymail.com       | secret   | Katie     | Secret
(1 row)

# select * from forum;
 forum_id | forum_name | group_id | forum_parent 
----------+------------+----------+--------------
         1|  1st forum |        1 |            -1
(1 row)

# select * from groups;
 group_id |      group_name       |                group_description                | group_founder 
----------+-----------------------+-------------------------------------------------+---------------
        1 | Java programming      | System.out.println("Hello world in Java!"); :)  |             1
(1 row)

I have these 3 tables in my database. I would like to get forums from it that are created by the user using id = 1

. How to do it? (in the table groups

, the id

user who created such a group is called group_founder

).

I mean, with the output above, you can see that user with id = 1

created a Java Programming group and then created a forum in that group called "1st Forum". Please, help:)

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2 answers


try it



SELECT *
FROM forum f
JOIN groups g ON g.group_id = f.group_id
JOIN users u ON u.user_id = g.group_founder 
WHERE u.user_id = 1
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According to your inputs, the query could be like this

SELECT U.login,U.email,U.firstname
FROM users U JOIN groups G ON U.user_id = G.group_founder
             JOIN forum F ON G.group_id = F.group_id
WHERE U.user_id = 1


DEMO

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