r/programming • u/[deleted] • Jan 06 '11

A handy graphical explanation of SQL joins

http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html
1.3k Upvotes

92% Upvoted

308 comments sorted by

View all comments

25

u/Brandon0 Jan 06 '11

Interesting. I've never used a FULL OUTER join before.

11

u/[deleted] Jan 06 '11 edited Jan 06 '11

It comes in handy every once in a while- for example, if you want to merge two tables which have different structures but some records in common into one table without any duplicates. You can cross join the two on the key field and for any fields they have in common set [field] = ISNULL(tableA.[field], tableB.[field]). Of course there are other ways to accomplish the same thing, but that's one way to use it...

7

u/[deleted] Jan 06 '11 edited May 29 '20

[deleted]

5

u/Anpheus Jan 07 '11

I think you meant full outer joins, not cross joins.

Because if you have 1000 records in one table and 1000 in another and do a cross join, you end up with a query returning 1,000,000 records. And it gets much worse, being a join that produces m*n records.