Set Operations

1. SQL has the set operations union, intersect and except.
2. Find all customers having an account.

    select distinct cname
   
   		 from depositor
   
   

3. union: Find all customers having a loan, an account, or both. branch.

    (select cname
   
   		 from depositor)
   
   		 union
   
   		 (select cname
   
   		 from borrower)
   
   

4. intersect: Find customers having a loan and an account.

    (select distinct cname
   
   		 from depositor)
   
   		 intersect
   
   		 (select distinct cname
   
   		 from borrower)
   
   

5. except: Find customers having an account, but not a loan.

    (select distinct cname
   
   		 from depositor)
   
   		 except
   
   		 (select cname
   
   		 from borrower)
   
   

6. Some additional details:
   • union eliminates duplicates, being a set operation. If we want to retain duplicates, we may use union all, similarly for intersect and except.
   • Not all implementations of SQL have these set operations.
   • except in SQL-92 is called minus in SQL-86.
   • It is possible to express these queries using other operations.