Question

I am trying to write an SQL query to retrieve all of a users pending events, however it is difficult with how my tables are structured.

My tables are as follows :

event {
event_id
name
group_id}

Pending {
GroupID
UserID
}

Users{
Username
UserID
}

Ever user is identified by a UserID, and every group by a GroupID. Events have in them a GroupID which points to a list of users. I need to retrieve all pending events for a certain user, so :

SELECT * FROM event
WHERE event.group_id = (SELECT GroupID FROM Pending)

But how do I then link this so only the Pending events for a user with a certain UserID are returned?

Answer 1

select e.* from event e
inner join pending p on
e.group_id = p.GroupID
inner join Users u
on p.UserID = u.UserID
where u.UserID = 123

actually you can skip the join with the Users-table if you already have the UserID:

select e.* from event e
inner join pending p on
e.group_id = p.GroupID
where p.UserID = 123

The typical way to write this is using inner join. Traditionally it has been better performing than sub selects, but modern DBMS:s optimize them into the same query. If you really want to write with a sub select you type like this

SELECT * FROM event
WHERE event.group_id in (SELECT GroupID FROM Pending WHERE UserID = 123)