How can I merge the columns from two tables into one output?

Question

I have two tables with similar information. Let\'s call them items_a and items_b. They should be one, but they are coming from different sources, s

Answer 1

When your are three tables or more, just add union and left outer join:

select a.col1, b.col2, a.col3, b.col4, a.category_id 
from 
(
    select category_id from a
    union
    select category_id from b
) as c
left outer join a on a.category_id = c.category_id
left outer join b on b.category_id = c.category_id