Select from one table where not in another

Question

I'm trying to find the rows that are in one table but not another, both tables are in different databases and also have different column names on the column that I'm using to match.

I've got a query, code below, and I think it probably works but it's way too slow:

SELECT `pm`.`id` FROM `R2R`.`partmaster` `pm` WHERE NOT EXISTS (     SELECT *      FROM `wpsapi4`.`product_details` `pd`     WHERE `pm`.`id` = `pd`.`part_num` ) 

So the query is trying to do as follows:

Select all the ids from the R2R.partmaster database that are not in the wpsapi4.product_details database. The columns I'm matching are partmaster.id & product_details.part_num

Answer 1

Expanding on Sjoerd's anti-join, you can also use the easy to understand SELECT WHERE X NOT IN (SELECT) pattern.

SELECT pm.id FROM r2r.partmaster pm WHERE pm.id NOT IN (SELECT pd.part_num FROM wpsapi4.product_details pd) 

Note that you only need to use ` backticks on reserved words, names with spaces and such, not with normal column names.

On MySQL 5+ this kind of query runs pretty fast.
On MySQL 3/4 it's slow.

Make sure you have indexes on the fields in question
You need to have an index on pm.id, pd.part_num.