Calculate the difference between results of two count(*) queries based on 2 tables in PostgreSQL

Try this way:

select 
  (
    SELECT 
      "count"(*) as val1 
    from 
      tab1
  ) - (
    SELECT 
      "count"(*) as val2 
    from 
      tab2
  ) as total_count

select t1.cnt - t2.cnt
from (select count(*) as cnt from tab1) as t1
  cross join (select count(*) as cnt from tab2) as t2

for same table you can do this.

select (count(abc)-count(DISTINCT xyz)) from tab;