Trying to do a three table join displaying all columns - sql

Question

I am trying to do a three table join to display all columns. Only two of the tables have a key in common. Below is the code for the tables I already have:
CREATE TABLE Faculty (
FacultyID int,
FirstName varchar(30),
LastName varchar(30),
EMail varchar(60),
BirthDate DATE,
Numbr_Courses int,
PRIMARY KEY (FacultyID)
);
CREATE TABLE Courses(
CourseID int,
CourseDisc varchar(4),
CourseNum varchar(4),
NumbrCred varchar(1),
FirstYrOffered int,
CourseTitle varchar(75),
PRIMARY KEY (CourseID)
);
CREATE TABLE Faculty_Courses(
InstanceID int,
FacultyID int,
CourseDisc varchar(4),
CourseNum varchar(4),
CourseTitle varchar(75),
PRIMARY KEY (InstanceID),
FOREIGN KEY (FacultyID) REFERENCES Faculty(FacultyID)
);
The two tables that have the same key is Faculty and Faculty_Course. I have attempted one peice of code that only gave me back an error. I guess I am having real trouble understanding how to do proper code for joining tables. My attempted code is below:
SELECT Faculty.*, Faculty_Courses.*
FROM Faculty INNER JOIN Courses
ON Faculty.FacultyID=Faculty_Courses.FacultyID
This gave me back the following error:
ERROR 1066 (42000): Not unique table/alias: 'Faculty'
Any help will be appreciated.

Answer 1

I would change your Faculty_Courses table:
CREATE TABLE Faculty_Courses(
InstanceID int,
FacultyID int,
CourseID int,
PRIMARY KEY (InstanceID),
FOREIGN KEY (FacultyID) REFERENCES Faculty(FacultyID),
FOREIGN KEY (CourseID) REFERENCES Courses(CourseID)
);
And then join all three tables together:
SELECT F.*, FC.*, C.*
FROM Faculty F
INNER JOIN Faculty_Courses FC ON F.FacultyID = FC.FacultyID
INNER JOIN Courses C ON C.CourseID = FC.CourseID
You probably only really want some of the columns from F (Faculty) and C (Courses) and can mostly ignore the FC columns as they are just used for mapping between F and C

Answer 2

I'm surprised your factory_courses table doesn't have a reference to courses.
Either way, with your current query, you are selecting from faculty and courses but aliasing faculty_courses -- you cannot do that. This is what your current query should look like:
select *
from faculty f
join faculty_courses fc on f.facultyid = fc.facultyid
To join the 3rd table, if you had a courseid in your faculty_courses table, perhaps something like this:
select *
from faculty f
join faculty_courses fc on f.facultyid = fc.facultyid
join courses c on fc.courseid = c.courseid
A Visual Explanation of SQL Joins

Answer 3

You're doing a join on two tables without actually linking two columns from those tables; joining Faculty and Courses using columns from Faculty and Faculty_Courses.
Since no column exists with unique values linking Courses with any other table, you cannot perform a join with Courses.
So you should join Faculty and Faculty_courses:
SELECT Faculty.*, Faculty_Courses.*
FROM Faculty INNER JOIN Faculty_Courses
ON Faculty.FacultyID=Faculty_Courses.FacultyID