在PostgreSQL中用多个表编写JOIN查询

Question

I've got 3 tables that look vaguely like this:

Users
----------
UserID
Name
Phone

User Groups
-----------
GroupID
GroupActivity

Group Membership
---------------
UserID
GroupID

I am trying to print the user name and phone number for all the groups with members undertaking a certain activity. For example, group 1 activity = knitting, group 2 activity = sewing and group 3 activity = knitting. I want to select all the members that are knitting but I am unsure how to join the three tables in one query. This is what I tried so far:

SELECT                                                                   
users.name, users.phone, groups.activity  
FROM users
INNER JOIN group_membership ON group_membership.userID = users.userID 
WHERE groups.activity = 'Knitting';

This ends up throwing the following error:

ERROR: missing FROM-clause entry for table "groups"

To fix this I tried adding teams to the FROM clause like so:

FROM users,groups

However, this in turn gives me this error:

ERROR: invalid reference to FROM-clause entry for table "members"

Any help here would be appreciated.

Answer 1

Try This

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';

Answer 2

You can have that shorter with table aliases and the USING clause:

SELECT ug.groupid, u.name, u.phone
FROM   user_groups ug
JOIN   group_membership USING (groupid)
JOIN   users u          USING (userid)
WHERE  ug.activity = 'knitting';

Details in the manual here.