Count the sum of a left outer join table

I have two tables which is

Student

ID	Name	Gender	Address
1	Abby	Girl	street
2	Mark	Boy	street
3	Lula	Girl	street
4	Bren	boy	street

Lessons

ID	Lessons_code
3	MK2234
5	22324KL
6	KCS233

and I want to join these tables then get the sum result of the students that didn’t took a lesson then group it by gender like this:

Gender	total
Boy	2
girl	1

I know it use sum() and left outer join (?) but I don’t know how to implement it.

Answer

I would suggest not exists:

select s.gender, count(*)
from students s
where not exists (select 1
                  from lessons l
                  where l.id = s.id
                 )
group by s.gender;

You have a very awkward data model. I would expect a column called lessons.id to refer to the primary key of the lessons table. Instead. it seems to refer to a student. A better name would be student_id.