Comments on: For those who don’t use SQL too much, just like me.

By: pkoch

pkoch — Thu, 19 Feb 2009 00:07:23 +0000

Hmm. I’ll run some benchmarks and post them.

About the style, it varies. NOT IN is sometimes more legible, sometimes it’s NOT EXISTS.

By: André Restivo

André Restivo — Wed, 18 Feb 2009 00:56:12 +0000

Imagine this query from the link you gave above:

SELECT DISTINCT store_type FROM stores
WHERE NOT EXISTS (SELECT * FROM cities_stores
WHERE cities_stores.store_type = stores.store_type);

For each line of the outer query the inner query has to be computed again because stores.store_type changes. Of course MySQL can have an intelligent caching mechanism, but if the number of stores is very big it won’t be enough. If MySQL is really intelligent it will transform the query into something similar to what I proposed. I know for a fact that PostgreSQL doesn’t like this type of queries very much.

In the example I wrote before. The inner query has to be performed only once and then it’s just a matter of using the index wisely.

Have you tried both approaches with some big chunks of data? You won’t notice the difference with small tables.

IMHO I think that the NOT IN clause is easier to understand than the NOT EXISTS clause and also more elegant. But it’s a matter of taste really …

By: pkoch

pkoch — Wed, 18 Feb 2009 00:24:33 +0000

Hmm, that’s contradictory to my MySQL experience. How/where did you observe that behaviour?

By: André Restivo

André Restivo — Tue, 17 Feb 2009 23:24:10 +0000

select * from category
where category.id not in (select cat_id from cat_post where post_id = $the_post_id)

This way seems easier and it’s probably faster as DB usually don’t cope well with ‘exists’ clauses.