Update and renew data based on data in other tables - sql

Question

There are 3 tables student, course, and takes as following
CREATE TABLE student
(
ID varchar(5),
name varchar(20) NOT NULL,
dept_name varchar(20),
tot_cred numeric(3,0) CHECK (tot_cred >= 0),
PRIMARY KEY (ID),
FOREIGN KEY (dept_name) REFERENCES department
ON DELETE SET NULL
)
CREATE TABLE takes
(
ID varchar(5),
course_id varchar(8),
sec_id varchar(8),
semester varchar(6),
year numeric(4,0),
grade varchar(2),
PRIMARY KEY (ID, course_id, sec_id, semester, year),
FOREIGN KEY (course_id, sec_id, semester, year) REFERENCES section
ON DELETE CASCADE,
FOREIGN KEY (ID) REFERENCES student
ON DELETE CASCADE
)
CREATE TABLE course
(
course_id varchar(8),
title varchar(50),
dept_name varchar(20),
credits numeric(2,0) CHECK (credits > 0),
PRIMARY KEY (course_id),
FOREIGN KEY (dept_name) REFERENCES department
ON DELETE SET NULL
)
tot_cred column data in the student table now is assigned with random values (not correct), I want to perform the query that updates and renews those data based on the course's grade each student has taken. For those students who received F grade will be excluded and those who didn't take any course will be assigned 0 as tot_cred.
I came up with two approaches, one is
UPDATE student
SET tot_cred = (SELECT SUM(credits)
FROM takes, course
WHERE takes.course_id = course.course_id
AND student.ID = takes.ID
AND takes.grade <> 'F'
AND takes.grade IS NOT NULL)
This query meets all my needs, but for those students who didn't take any course, it does assign NULL value instead of 0.
The second is using case when
UPDATE student
SET tot_cred = (select sum(credits)
case
when sum(credits) IS NOT NULL then sum(credits)
else 0 end
FROM takes as t, course as c
WHERE t.course_id = c.course_id
AND t.grade<>'F' and t.grade IS NOT NULL
)
But it assigned 0 to all students. Is any way to achieve the above requirement?

Answer 1

If the 1st query meets your requirement and the only problem is that it returns NULL for the students that did not take any course then the easiest solution would be to use instead of SUM() aggregate function the function TOTAL() which will return 0 instead of NULL:
UPDATE student AS s
SET tot_cred = (
SELECT TOTAL(c.credits)
FROM takes t INNER JOIN course c
ON t.course_id = c.course_id
WHERE t.ID = s.ID AND t.grade <> 'F' AND t.grade IS NOT NULL
);
The same could be done with COALESCE():
SELECT COALESCE(SUM(credits), 0)...
Also, use a proper join with an ON clause and aliases for the tables to improve readability.