Question

Assuming tables A and B, how do I write a query that would return all columns of A and a flag indicating the presence of a corresponding child row in B

For example:

col1_a|col2_a|…|hasrowinb

Answer 1

Perhaps the following is the “simplest” way of doing the query, while guaranteeing that there are no duplicates:

select a.*,
       (case when 0 = (select count(*) from b where b.aid = a.aid)
             then 'N'
             else 'Y'
        end) as hasRowInB
from a