--Below is an excerpt of the relational algebra operations implemented in SQL

--difference
select * from customer
except
select * from customer where repnum = 30

--intersect
select * from customer
intersect
select * from customer where repnum = 30

--product
select * from customer cross join rep

--union
select customernum,customername from customer
union
select customernum from customer where repnum = 30

--union all
select * from customer
union all
select * from customer where repnum = 30