Difference between inner and outer joins

[James Adams]

Can someone explain the difference between inner and outer joins in SQL queries? Is there an online reference?

Can someone suggest a better forum/newsgroup for general SQL questions?

Thanks in advance.


--James

[Paul Sturrock]

An inner join is just a join between tables where the rows returned are only those which satisfy the join condition. So:

SELECT * FROM table_a, table_b 
WHERE table_a.id = table_b.table_a_id

would be an inner join.

An outer join is one which returns all the rows you would get from an inner join, plus some from one table which do not satisfy the join condition. Suppose you did this:

SELECT table_a.id, table_b.name 
FROM table_a LEFT OUTER JOIN table_b ON
table_a.id = table_b.table_a_id

You would get all the records our first join gives us (i.e. everything where table_a.id = table_b.table_a_id) plus all table_a records whose id does not appear in table_b's table_a_id attribute. Does that make any sense?

(And don't worry - general SQL questions often appear and are answered in here)
[ June 23, 2005: Message edited by: Paul Sturrock ]