Help with a WHERE on a LEFT JOIN SQL Query

Question

I\'m trying to construct a query that will include a column indicating whether or not a user has downloaded a document. I have a table called HasDownloaded with the followin

Answer 1

@Mark: I understand why the JOIN syntax works, but thanks for the warning. I do think your suggestion is more intuitive. I was curious to see which was more efficient. So I ran a quick test (this was rather simplistic, I'm afraid, over only 14 rows and 10 trials):

In the JOIN condition:

AND HasDownloaded.memberID = @memberID

• Client processing time: 4.6
• Total execution time: 35.5
• Wait time on server replies: 30.9

In the WHERE clause:

WHERE HasDownloaded.memberId IS NULL OR HasDownloaded.memberId = @memberId

• Client processing time: 7.7
• Total execution time: 27.7
• Wait time on server replies: 22.0

It looks like the WHERE clause is ever-so-slightly more efficient. Interesting! Once again, thanks to both of you for your help.

Answer 2

WHERE HasDownloaded.memberId IS NULL OR HasDownloaded.memberId = @memberId

would be the normal way to do that. Some would shorten it to:

WHERE COALESCE(HasDownloaded.memberId, @memberId) = @memberId

You can, as Matt B. shows, do it in your JOIN condition - but I think that's much more likely to confuse folks. If you don't understand WHY moving it to the JOIN clause works, then I'd strongly suggest staying away from it.

Answer 3

Move the condition in the WHERE clause to the join condition.

SELECT Documents.name, HasDownloaded.id FROM Documents
LEFT JOIN HasDownloaded ON HasDownloaded.documentID = Documents.id 
  AND HasDownloaded.memberID = @memberID 

This is necessary whenever you want to refer to a left join-ed table in what would otherwise be the WHERE clause.