how does the count function in mysql work? - mysql

Question

First and foremost, this is part of an assignment, but I'm trying to get more clarification on how the count() function works when querying a db.
The question is: List the name, SSN and the number of courses the student has taken (courses with the same CourseNumber taken in different quarters are counted as different courses).
I've been querying using this:
SELECT S.Name, S.SSN, COUNT(*)
FROM Student S, Enrollment E
WHERE S.SSN = E.SSN
GROUP BY S.SSN
which seems to return the correct answer, but I'm not sure why. As a result, I can't seem to get the next questions (assuming courses with the same CourseNumber taken in different quarters are considered as one course) correct.
Here are the create table commands so you can see which table holds what info:
CREATE TABLE Student(
SSN INT(9),
Name VARCHAR(20),
Major VARCHAR(30),
PRIMARY KEY (SSN)
);
CREATE TABLE Course(
CourseNumber INT(5),
PrerequisiteCourseNumber INT(10),
CourseTitle VARCHAR(10),
NumberUnits INT(2),
PRIMARY KEY (CourseNumber)
);
CREATE TABLE Section(
CourseNumber INT(5),
Quarter VARCHAR(10),
RoomNumber INT(5),
DayTime VARCHAR(20),
PRIMARY KEY (CourseNumber,Quarter),
FOREIGN KEY (CourseNumber) REFERENCES Course(CourseNumber)
);
CREATE TABLE Enrollment(
SSN INT(9),
CourseNumber INT(5),
Quarter VARCHAR(10),
Grade VARCHAR(1),
PRIMARY KEY (SSN,CourseNumber,Quarter),
FOREIGN KEY (SSN) REFERENCES Student(SSN),
FOREIGN KEY (CourseNumber) REFERENCES Course(CourseNumber),
FOREIGN KEY (Quarter) REFERENCES Section(Quarter)
);
Any pointers?

Answer 1

what you are doing is using old join syntax (pre ansi syntax) where you do a cross join of tables and use the where to turn it into a join. an equivalent query would be this.
SELECT S.Name, S.SSN, COUNT(*)
FROM Student S
JOIN Enrollment E ON S.SSN = E.SSN
GROUP BY S.SSN
Now to answer your question about what the count is doing and stuff..
COUNT() returns a count of every row that is returned.
GROUP BY groups all of the records by a common ground aka your SSN field. so if a student has 5 SSN rows in the table then his count will be 5.
Now for the part you are looking for with course numbers.. you need to JOIN those tables on appropriate fields and add a field to your group by for each quarter
aka add to the previous code..
WHERE E.quarter = whatever_quarter_you_want
you can add more to this query if you need to. but add data to your question if you want a more full answer.

Answer 2

Your current query is:-
SELECT S.Name, S.SSN, COUNT(*)
FROM Student S
INNER JOIN Enrollment E
ON S.SSN = E.SSN
GROUP BY S.SSN
What this is doing is joining student to enrollment, so giving multiple rows for each student, one for each course / quarter they are enrolled on. It is then grouped by SSN (student number?) to count up the number of course / quarters they are enrolled on. You should really group by S.Name as well (while MySQL won't object, most flavours of SQL would).
Note that COUNT(*) counts the number of rows. You could use COUNT(E.CourseNumber) which would count the number of rows where course number is not null. Not really useful here, but can be useful with LEFT OUTER JOINs. You can also use COUNT(DISTINCT CourseNumber) to count the number of unique non null course numbers for the student.
A LEFT OUTER JOIN might also be better as this would enable you to return 0 as the counts for students who exist but who are not enrolled in any courses:-
SELECT S.Name, S.SSN, COUNT(DISTINCT E.CourseNumber)
FROM Student S
LEFT OUTER JOIN Enrollment E
ON S.SSN = E.SSN
GROUP BY S.Name, S.SSN

Answer 3

SELECT S.Name, S.SSN, COUNT(*)
FROM Student S, Enrollment E, Section Sec, Course C
WHERE S.SSN = E.SSN AND E.CourseNumber = Sec.CourseNumber
AND Sec.CourseNumber = C.CourseNumber AND Sec.Quarter like E.Quarter
GROUP BY S.Name, S.SSN