Question

I have the next table: code, year, day

I want to find the code of the courses that was taken in 2000 in Sunday and Monday.

So I wrote the next query:

Select DISTINCT dbo.Course.Code
From dbo.Course
Where dbo.Course.CourseYear = 2000 AND 
EXISTS (Select * From dbo.course Where (dbo.course.day = 'Sunday' and dbo.Course.CourseYear = 2000))and 
EXISTS (Select * From dbo.course Where (dbo.course.day = 'Monday' and dbo.Course.CourseYear = 2000))

It’s return me the courses that were given in 2000, in Sunday or Monday. Why?

Answer 1

You need to enforce that the same course record is being checked in both exists statement.
Your exists statement only ensure that there exists any course that was offered in sunday and any course that was offered on monday

Revising your query:

Select DISTINCT dbo.Course.Code
From dbo.Course c
Where dbo.Course.CourseYear = 2000 AND 
EXISTS (Select * From dbo.course Where (dbo.course.day = 'Sunday' and dbo.Course.CourseYear = 2000 and dbo.course.Code = c.Code))and 
EXISTS (Select * From dbo.course Where (dbo.course.day = 'Monday' and dbo.Course.CourseYear = 2000 and dbo.course.Code = c.Code))

A better written version

select distinct c.code
from
dbo.course c
inner join dbo.course c2 on c.code = c2.code
where
c.CourseYear = 2000
and c2.CourseYear = 2000
and c.Day = 'Monday'
and c2.Day = 'Sunday'

Also note that your schema is not normalized. You should have two tables:

1. Course (CourseId, Code)
2. CourseInstance (CourseId, Day, Year)