MySQL: difference of two result sets

Question

How can I get the set difference of two result sets?

Say I have a result set (just one column in each):

result1:
\'a\'
\'b\'
\'c\'

result2:
\'b\'
\'         


        

Answer 1

I recently had this requirement where I had to find the difference between two resultsets. Although the above answers helped me wished they were a little detailed. For the given question I found two interpretation:

1. The resultset could be from 2 different tables
2. The resultset from the same table

For the first one where the resultset can be from 2 different tables, let's take two tables: science_student and math_student.

science_student

math_student

I want to calculate the difference between these 2 resultsets, that is:

result1 - result2

result1: select student_id from science_student where id > 2

result2: select student_id from math_student

The difference between result1 - result2 is STUD3

So the query to find the difference will be:

select result1.student_id 
 from 
 (select student_id from science_student where id > 2) result1
 left join
 (select student_id from math_student) result2
 on result1.student_id = result2.student_id 
 where result2.student_id is null;

For the second interpretation where the resultset can be from the same table:

result1 - result2

result1: select student_id from science_student 

result2: select student_id from science_student where id > 2

The difference between result1 - result2 is STUD1, STUD2

And the query for the same will be:

select result1.student_id 
 from 
 (select student_id from science_student) result1
 left join
 (select student_id from science_student where id > 2) result2
 on result1.student_id = result2.student_id 
 where result2.student_id is null;