Using binary logic in PostgreSQL JOIN queries - sql

Question

I've got 3 tables that look vaguely like this:
Users
----------
UserID
Name
Phone
User Groups
-----------
GroupID
Activity
Group Membership
---------------
UserID
GroupID
Independent Actives
-------------------
UserID
Activity
The idea is that a user can perform an activity either as part of a group or on their own. What I want to do is return all the people that partake in a certain activity. What I have been able to write so far lets me return all the users which are in groups that undertake that activity. What I want to add to this is the ability to see the people that do the activity independently. This is what I have so far:
SELECT
users.name, users.phone, user_groups.activity
FROM users
INNER JOIN group_membership ON group_membership.userID = users.userID
INNER JOIN user_groups ON user_groups.groupID = group_membership.groupID
WHERE user_groups.activity = 'Knitting';
The above bit works fine and it shows all of the users that are part of groups that do knitting, but I also want it to show all the users that are knitting independently. This is what I have attempted to add:
SELECT
users.name, users.phone, user_groups.activity
FROM users
INNER JOIN group_membership ON group_membership.userID = users.userID
INNER JOIN user_groups ON user_groups.groupID = group_membership.groupID
INNER JOIN independent_activity ON independent_activity.userID = users.userID
WHERE user_groups.activity = 'Knitting' OR independent_activity.activity = 'Knitting';
The problem here is the syntax, I understand the algorithm that I'm trying to do but I don't know how to transfer it into sql and so any help is appreciated.

Answer 1

You could use a UNION in this case
SELECT users.NAME
,users.phone
,user_groups.activity
FROM users
INNER JOIN group_membership ON group_membership.userID = users.userID
INNER JOIN user_groups ON user_groups.groupID = group_membership.groupID
WHERE user_groups.activity = 'Knitting'
UNION
SELECT users.NAME
,users.phone
,independent_activity.activity
FROM users
INNER JOIN independent_activity ON independent_activity.userID = users.userID
WHERE independent_activity.activity = 'Knitting';
You also might want to lookup the differences between a UNION and a UNION ALL and decide the one that suites your requirement.

Answer 2

You've got a working answer from SoulTrain. However, for completeness sake I'd like to mention that you don't have to join all those tables. (You could use outer joins here and remove duplicate matches with DISTINCT, but that's not necessary. You don't have to query the users table twice either. And you don't need UNION for doing the distinct job.)
Simply select from the one table you want to display data from, i.e. the users table, and then use EXISTS or IN to get only those users that are either in one set or another.
select name, phone
from users
where userid in
(
select userid
from independent_actives
where activity = 'Knitting'
)
or userid
(
select userid
from group_membership
where groupid in (select groupid from user_groups where activity = 'Knitting')
)