SQL statement that shows at least 16 courses not taken - mysql

Question

I have four schemas:
takes(ID,course_id,sec_id,semester,year)
student(ID,name,dept_name,tot_credit)
course(course_id,title,dept_name,credits)
department(dept_name,building,budget)
I want to create a query that finds the name and id of each Astronomy student whose name begins with the letter ’T’ and who has not taken at least 16 Astronomy courses.
What's the easiest way I could do this?
I already wrote this beginning bit
SELECT name, id
FROM student
WHERE dept_name='Astronomy' AND name LIKE '%T%'
I'm not quite sure how to finish this off.
Any help would be greatly appreciated :)
Here's the result
NAME ID CLASS_TAKEN
-------------------- ----- -----------
Tolle 38279 12
Teo 62268 13
Tolle 93223 13
Tsukamoto 17707 5
Titi 11576 9
Teo 91772 12
Toraichi 50387 11
Tewari 80754 14
Tiroz 64091 14
9 rows selected
I need Teo with the id 91772 and Tewari 80754 to be gone

Answer 1

Given my reading of the requirements and the comments it's pretty clear that the question is not very clear. :-) What you're looking for are students where (total # of courses given by the Astronomy department) - (# of Astronomy courses taken by student) >= 16. So, how do we find these values? First, let's start with the total number of courses given by the Astronomy department. This is pretty simple:
SELECT COUNT(*) AS ASTRONOMY_COURSE_COUNT
FROM COURSE
WHERE DEPT_NAME = 'ASTRONOMY'
Now, the second part is to determine how many courses given by the Astronomy department each student has taken. To do this we need to start with the student, join to the courses the student has taken (the TAKES table), then join to the COURSES table to find out which department each course is part of. Something like the following should do it:
SELECT s.ID, s.NAME, COUNT(*) AS STUDENT_ASTRO_COUNT
FROM STUDENT s
INNER JOIN TAKES t
ON t.ID = s.ID
INNER JOIN COURSE c
ON c.COURSE_ID = t.COURSE_ID
WHERE c.DEPT_NAME = 'ASTRONOMY' AND
s.NAME LIKE 'T%'
GROUP BY s.ID, s.NAME;
OK, now we need to put this together. You've tagged this question for both Oracle and MySQL so I'm going to guess you'll accept valid syntax for either database; thus I'll use Oracle Common Table Expression syntax to pull everything together:
WITH ASTRONOMY_COURSES AS (SELECT COUNT(*) AS ASTRONOMY_COURSE_COUNT
FROM COURSE
WHERE DEPT_NAME = 'ASTRONOMY'),
STUDENT_ASTRO_COURSES AS (SELECT s.ID,
s.NAME,
COUNT(*) AS STUDENT_ASTRO_COUNT
FROM STUDENT s
INNER JOIN TAKES t
ON t.ID = s.ID
INNER JOIN COURSE c
ON c.COURSE_ID = t.COURSE_ID
WHERE c.DEPT_NAME = 'ASTRONOMY' AND
s.NAME LIKE 'T%'
GROUP BY ID)
SELECT s.ID,
s.NAME,
s.STUDENT_ASTRO_COUNT,
a.ASTRONOMY_COURSE_COUNT - s.STUDENT_ASTRO_COUNT AS UNTAKEN_COUNT
FROM STUDENT_ASTRO_COURSES s
CROSS JOIN ASTRONOMY_COURSES a
WHERE a.ASTRONOMY_COURSE_COUNT - s.STUDENT_ASTRO_COUNT >= 16;
Note here that a CROSS JOIN is used to put together the subqueries. This means that all the rows of each subquery are joined to all the rows of the other subquery - but since in this case the ASTRONOMY_COURSES subquery will only return a single row what we're doing is appending the ASTRONOMY_COURSE_COUNT value onto each row returned by the STUDENT_ASTRO_COURSES subquery.
That should at least get you pretty close. Amend as needed.
Not tested on animals - you'll be first! :-)
Share and enjoy.

Answer 2

Do you need to use all tables?
Table department has no links with the student,
Table takes has no links with the student,
Table coursehas no links with the student.
If student lists total credits that are all Astronomy I think this can be used:
select name, id, MAX(tot_credit) as credits
from student
where dept_name='Astronomy' and name like 'T%'
group by name, id
having MAX(tot_credit)<=16
PS - you schema is not good; PK-FK references are missing

Answer 3

Your query will need to reference more tables than just the student table.
Your tables seem be missing some important information, which student has taken which course. There's a table named takes, but there doesn't appear to be any relationship between takes and student.
So first, figure out how to list the students along with the Astronomy courses they have taken. Each row will identify the student and a course.
SELECT s.id AS student_id
, s.name AS student_name
, t.???
FROM student s
JOIN ??? t
ON t.student_id = s.id
WHERE ...
You may also need to include another "join" to an additional table, in order to identify which course a student has taken is an Astronomy course.
To also include students that have not take any Astronomy courses, you can use an outer join, rather than an inner join. (That would mean including the LEFT keyword before JOIN, and relocating predicates from the WHERE clause to the ON clause. (A predicate in the WHERE clause that can only be satisfied by non-NULL values will negate the outer-ness of the join.)
Once you have a query that returns that set (students along with any astronomy courses they've taken), you can then add a GROUP BY clause to "collapse" a set of rows into a single row. (Looks like you want the rows "grouped" by student.)
And then an aggregate function like COUNT() or SUM() can be used to get a count of rows for each group. (If you don't want to count any re-takes of a course (a "duplicate" course for a student) you may be able to make use of the COUNT(DISTINCT t.foo) form.
And then a HAVING clause can be added to the query, to compare the value returned from the aggregate expression to a constant value, to return only rows that satisfy a specific condition.
FOLLOWUP
Assuming:
CREATE TABLE course
( id INT UNSIGNED NOT NULL COMMENT 'PK'
, title VARCHAR(30) NOT NULL COMMENT 'course title'
, dept_name VARCHAR(30) NOT NULL COMMENT 'FK ref dept.name'
, credits DECIMAL(5,2) COMMENT 'credit hours'
, PRIMARY KEY (id)
);
CREATE TABLE student
( id INT UNSIGNED NOT NULL COMMENT 'PK'
, name VARCHAR(30) NOT NULL COMMENT 'student name'
, dept_name VARCHAR(30) NOT NULL COMMENT 'FK ref dept.name'
, tot_credit INT COMMENT '?'
, PRIMARY KEY (id)
);
CREATE TABLE takes
( student_id INT UNSIGNED NOT NULL COMMENT 'FK ref student.id'
, course_id INT UNSIGNED NOT NULL COMMENT 'FK ref course.id'
, sec_id INT UNSIGNED NOT NULL COMMENT '?'
, semester INT UNSIGNED NOT NULL COMMENT '?'
, year INT UNSIGNED NOT NULL COMMENT '?'
, PRIMARY KEY (student_id, course_id, sec_id, semester, year)
, CONSTRAINT FK_takes_course FOREIGN KEY (course_id) REFERENCES course (id)
, CONSTRAINT FK_takes_student FOREIGN KEY (student_id) REFERENCES student (id)
);
Query to get a list of students...
SELECT s.id
, s.name
FROM student s
WHERE s.name LIKE 'T%'
AND s.dept_name = 'ASTRONOMY'
Get list of students along with the courses they've taken, returning the id of the ASTRONOMY courses they've taken...
SELECT s.id AS student_id
, s.name AS student_name
, c.id AS course_id
FROM student s
LEFT
JOIN takes t
ON t.student_id = t.id
LEFT
JOIN course c
ON c.id = t.course_id
AND c.dept_name = 'ASTRONOMY'
WHERE s.name LIKE 'T%'
AND s.dept_name = 'ASTRONOMY'
Collapse the rows to one per student using a GROUP BY, and use aggregate functions to get counts or totals.
SELECT s.id AS student_id
, s.name AS student_name
, SUM(c.credits) AS total_astronomy_credits_taken
, COUNT(c.id) AS count_astronomy_courses_taken
, COUNT(DISTINCT c.id) AS count_distinct_astronomy_courses_taken
FROM student s
LEFT
JOIN takes t
ON t.student_id = t.id
LEFT
JOIN course c
ON c.id = t.course_id
AND c.dept_name = 'ASTRONOMY'
WHERE s.name LIKE 'T%'
AND s.dept_name = 'ASTRONOMY'
GROUP
BY s.id
, s.name
To omit rows from this resultset, add a HAVING clause. For example, to exclude rows where total_astronomy_credits_taken is greater than or equal to 12...
HAVING total_astronomy_credits_taken >= 12
If you want the rows returned in a certain sequence, specify that in an ORDER BY clause
ORDER BY s.id
If you want to replace NULL values from the aggregates with zeroes, you can warp the aggregate expression in an IFNULL(foo,0) function, e.g.
, IFNULL(COUNT(c.id),0) AS count_astronomy_courses_taken

Answer 4

Try this :
select a.name, a.id, count(b.ID) as class_taken
from student a inner join takes b
on a.ID = b.ID
inner join course c
on b.course_id = c.course_id
where a.dept_name='Astronomy' and substring(a.name,1,1) = 'T'
group by a.name, a.id
having count(b.ID) < 17