Question

I have a post/tag database, with the usual post, tag, and tag_post tables. The tag_post table contains tagid and postid fields.

I need to query posts. When I want to fetch posts that have a certain tag, I have to use a join:

... INNER JOIN tag_post ON post.id = tag_post.postid 
WHERE tag_post.tagid = {required_tagid}`

When I want to fetch posts that have tagIdA and tagIdB, I have to use two joins (which I kind of came to terms with eventually).

Now, I need to query posts that do not have a certain tag. Without much thought, I just changed the = to !=:

... INNER JOIN tag_post ON post.id = tag_post.postid 
WHERE tag_post.tagid != {certain_tagid}`

Boom! Wrong logic!

I did come up with this – just writing the logic here:

... INNER JOIN tag_post ON post.id = tag_post.postid 
WHERE tag_post.postid NOT IN 
(SELECT postid from tag_post where tagid = {certain_tagid})

I know this will work, but due to the way I’ve been brought up, I feel guilty (justified or not) whenever I write a query with a subquery.

Suggest a better way to do this?

Answer 1

You can think of it as “find all rows in posts that do not have a match in tags (for a specific tag)”

This is the textbook use case for a LEFT JOIN.

LEFT JOIN tag_post ON post.id = tag_post.postid AND tag_post.tagid = {required_tagid}
WHERE tag_post.tag_id IS NULL

Note that you have to have the tag id in the ON clause of the join.

For a reference on join types, see here: http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html