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

by

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.

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