Question

I am doing an except query in SQL like this

Q1 EXCEPT Q2 EXCEPT Q3

Where Q1, Q2 and Q3 are sub-query.
I just want to know what will be its output, (Q1-Q2)-Q3 or Q1-(Q2-Q3)? and if 2nd, how to get 1st one as output?

Answer 1

I don’t have an installation of DB2, but in PostgreSQL, Q1 except Q2 except Q3 appears to be interpolated as (Q1 except Q2) except Q3 (note: generate_series(m,n) is a function found in PostgreSQL that generates a single column of integer values from m to n (where m<n, of course)):

select generate_series(1,10) except select generate_series(5,15) except select generate_series(10,20);


 generate_series
-----------------
               1
               2
               3
               4
(4 rows)

select * from (select generate_series(1,10) except select generate_series(5,15))a except select generate_series(10,20);


 generate_series
-----------------
               1
               2
               3
               4
(4 rows)

select generate_series(1,10) except select * from (select generate_series(5,15) except select generate_series(10,20))a;


 generate_series
-----------------
               1
               2
               3
               4
              10
(5 rows)

However, it’s best to use parentheses to make certain that the order of evaluation is how you want it.