Creating query that returns id if condition is matched in rows from two tables

Question

I\'m learning SQL/dbms and using Postgres. I want to return rows that both have a certain value in a certain column. For example in tables Carpets and Curtain

Answer 1

Since the results are for two disjoint tables, you actually want a union instead:

select id, 'curtains' as source from curtains where color = 'lightyellow'
  union 
select id, 'carpets' as source from carpets where color = 'lightyellow'

Regarding joins, learn them all together. They are just slight variations of each other.