evaluate condition

Question

I have 3 tables: Course, Professor, Degree.

The course table lists all courses being taught and the professor who is assigned to teach it using the professor ID. The professor table lists all the professors along with their general contact information. It's primary key is the professor ID. The degree table lists all the degrees held by professors using their professor ID as a foreign key.

The problem I am having is that when I want a listing of each course along with the assigned professor and whether that professor has a doctorate degree. If they do return a “Y” if not return an “N”. I only want one result per professor despite there being multiple entries for each professor in the table.

Sample Table 1

ProfessorID Professor
01          Mary Smith
02          Harry Johnson
03          Teddy Fitz
04          Larry Manson
05          Sarah Love


CourseID    CourseName   ProfessorID
001         Math 101     01
002         English 201  01
003         English101   02
004         Math 101     03
005         Science 101  04

Sample Table 2

DegreeID    Degree  Doctorate Degree    ProfessorID
0001        BA      N                   01
0002        MS      N                   01
0003        PHD     Y                   01
0004        BA      N                   02
0005        MS      N                   02
0006        BA      N                   03
0007        MS      N                   03
0008        EDD     Y                   03
0009        BA      N                   04

The Results I would like is:

Course       Professor      Doctorate
Math 101     Mary Smith     Y
English 201  Mary Smith     Y
English101   Harry Johnson  N
Math 101     Teddy Fitz     Y
Science 101  Larry Manson   N

However, I keep getting a result for each degree that the professor has; regardless of whether it is a doctorate or not. Such as the following:

Course      Professor       Doctorate
Math 101    Mary Smith      Y
Math 101    Mary Smith      N
English 201 Mary Smith      Y
English 201 Mary Smith      N
English101  Harry Johnson   N
Math 101    Teddy Fitz      Y
Science 101 Larry Manson    N

How should I code this?

Answer 1

There are different ways of fixing it. Given the final result table, you can use a statement similar to the following to get the result you are looking for:

SELECT Course, Professor, MAX(Doctorate) as Doctorate 
FROM YOUR_RESULTING_TABLE 
GROUP BY Course, Professor

the MAX() function on letters return based on the lexicographic(alphabetical) order.

Answer 2

There are lots of ways to achieve what you want. I'll site 2 examples:

You can use LEFT JOIN + COALESCE , since those professor without doctorate can be default to 'N':

SELECT 
A.CourseName,
B.Professor,
COALESCE(C.Doctorate, 'N')
FROM course_table as A
LEFT JOIN professor_table as B
ON A.ProfessorID = B.ProfessorID
LEFT JOIN degree_table as C
ON B.ProfessorID = C.ProfessorID AND C.Doctorate = 'Y'

UPDATED: