Explanation on how the minus/except operator works - sql

Question

I am trying to figure out how the minus/except operator works.
Somehow I cannot find anything useful on the web. The "minus" operator is used to return all rows in the first statement that are not part of the second statement. But how exactly does it manage to do this?
Can someone please provide me how this is done "step by step"?

Answer 1

Note: The following answer is true for Oracle. See The UNION [ALL], INTERSECT, MINUS Operators
My SQL has only UNION and UNION ALL. Results for INTERSECT and MINUS can be got using IN and NOT IN. See Union, Difference, Intersection, and Division in MySQL [PDF]
In SQL Server, MINUS is called EXCEPT. See Set Operators (Transact-SQL)
I am surprised you are unable to find anything related to this on the Web. MINUS is a set operation in SQL, others include UNION, UNION ALL and INTERSECT.
This is what they do:
Sample Data:
EMPLOYEE
ID NAME SALARY AGE
1 Alice 5000 23
2 Joe 1000 25
3 Raj 2000 28
4 Pam 1500 32
UNION:
Returns results from SQL 1 combiled with Results from SQL 2, after removing duplicates. A variation is UNION ALL that does not remove duplicates. UNION ALL has better performance because it does not do the sort and remove duplication (internal) step. Union all is useful when the results of two SQLs being used are mutually exclusive.
select * from employee where salary > 1000
union
select * from employee where age > 25
returns all employees that are 25 years old or more or have a salary > 1000 (satisfy either condition)
ID NAME SALARY AGE
1 Alice 5000 23
3 Raj 2000 28
4 Pam 1500 32
Using UNION ALL in the above case returns record for Raj twice because UNION ALL does not remove duplicates.
select * from employee where salary > 1000
union all
select * from employee where age > 25
ID NAME SALARY AGE
1 Alice 5000 23
3 Raj 2000 28
4 Pam 1500 32
3 Raj 2000 28
INTERSECT:
Returns only common records between the result sets.
select * from employee where salary > 1000
intersect
select * from employee where age > 25
returns only those records that satisfy both conditions: Have salary > 1000 AND are over 25.
ID NAME SALARY AGE
3 Raj 2000 28
4 Pam 1500 32
MINUS:
Returns records from SQL 1 after removing results from SQL 2:
select * from employee where salary > 1000
intersect
select * from employee where age > 25
returns all those employees that have a salary > 1000 after removing employees that are more than 25 years of age:
ID NAME SALARY AGE
1 Alice 5000 23

Answer 2

Assume:
create table U
( x int not null primary key );
insert into U (x) values (1),(2),(3);
create table V
( y int not null primary key );
insert into U (x) values (3),(4);
Now U - V = { 1, 2 }
U - V can be expressed as:
All tupels in U that does not exist in V, i.e.
select x
from U
where not exists (
select 1
from V
where V.y = U.x
);
In the same way V - U = { 4 }
Did that clarify?

Answer 3

this should give you a clue !
create table #t(id int)
insert into #t values(1),(2),(3),(4),(5)
create table #t2(id1 int)
insert into #t2 values(2),(5),(6)
select * from #t except select * from #t2
select id from #t left join #t2 on #t.id=#t2.id1 where #t2.id1 is null
SEE DEMO
IN set theory
say A={A,B,C,D} ,B={B,X,Y,Z}
so A-B={A,C,D} -- which is a left join in sql