Week 08 Lectures (64)

Subqueries ⇒ convert to a join

Example: (on schema Courses(id,code,...), Enrolments(cid,sid,...), Students(id,name,...)

select c.code, count(*)
from   Courses c
where  c.id in (select cid from Enrolments)
group  by c.code

becomes

select c.code, count(*)
from   Courses c join Enrolments e on c.id = e.cid
group  by c.code