I have two tables with similar information. Let's call them
Specifying the columns on your query should do the trick:
select a.col1, b.col2, a.col3, b.col4, a.category_id from items_a a, items_b b where a.category_id = b.category_id
should do the trick with regards to picking the columns you want.
To get around the fact that some data is only in items_a and some data is only in items_b, you would be able to do:
select coalesce(a.col1, b.col1) as col1, coalesce(a.col2, b.col2) as col2, coalesce(a.col3, b.col3) as col3, a.category_id from items_a a, items_b b where a.category_id = b.category_id
The coalesce function will return the first non-null value, so for each row if col1 is non null, it'll use that, otherwise it'll get the value from col2, etc.