SQL Query trouble - sql

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)

Answer 2

You want to use the JOIN clause to link the tables together rather than using the WHERE clause to filter
SELECT u.username,
e.name
FROM users u
INNER JOIN pending p
ON u.userid = p.userid
INNER JOIN event e
ON p.groupid = e.groupid
WHERE
u.UserID = SomeID

Answer 3

SELECT
event.event_id
FROM event
LEFT JOIN Pending ON event.group_id = Pending.GroupID
LEFT JOIN Users USING (UserID)
WHERE Users.Username = 'foobar'
USING() can be utilized when columns on both sides of selection have the same name. It makes for easier reading. That is one of main reason why i would recommend to have same name for same data throughout the database. For example, if you Documents table and primary key document_id, then in all the other tables, where you are referencing the ID of a document, you use the same name for the column.
To learn more about JOINs : in mysql or postgresql read the links. And for visual representation of what each join does: this article.
Also you should get some book about your preferred RDBMS and learn all the basics, then you can expand your knowledge by reading SQL Antipatterns book. Or you could just carefully look through slides, made by book's author.

Answer 4

You can try :
select e.*, u.* from pending as p, users as u, event as e
WHERE
p.group_id = e.group_id
AND p.user_id = u.userID

Answer 5

SELECT * FROM event E,
pending P,
Users U
WHERE E.group_id = P.GroupID
AND P.UserID = U.UserID
AND U.UserID = (Some XYZ id)
XYZ should be some Integer value.