IS NULL not finding all null values in column MYSQL

Question

UPDATE student as s
LEFT JOIN takes as t ON s.ID = t.ID
LEFT JOIN course as c ON t.course_id = c.course_id
SET s.tot_cred = s.tot_cred - COALESCE(c.credits, 0)
WHERE t.grade = 'F'
OR
t.grade IS NULL

I am trying to update tot_cred in student by subtracting the credit value of any class the student has failed, grade in takes relation = 'F', or is currently taking, grade in takes relation IS NULL.

The above query works for the single 'F' grade in the 'takes' relation. However it will only work for one of the 4 NULL values in the 'takes' relation in the grade column.

Pretty new to SQL and I feel that something in the query is only looking for the first NULL value and not all of them.

Tables:

mysql> select ID, tot_cred
    -> from student;
+-------+----------+
| ID    | tot_cred |
+-------+----------+
| 00128 |      102 |
| 12345 |       32 |
| 19991 |       80 |
| 23121 |      110 |
| 44553 |       56 |
| 45678 |       38 |
| 54321 |       54 |
| 55739 |       38 |
| 70557 |        0 |
| 76543 |       58 |
| 76653 |       60 |
| 98765 |       98 |
| 98988 |      112 |
+-------+----------+

I want to change the tot_cred here if the corresponding grade is F or NULL

mysql> select ID, course_id, grade
    -> from takes
    -> order by grade;
+-------+-----------+-------+
| ID    | course_id | grade |
+-------+-----------+-------+
| 76543 | CS-001    | NULL  |
| 54321 | CS-001    | NULL  |
| 12345 | CS-001    | NULL  |
| 98988 | BIO-301   | NULL  |
| 98988 | BIO-101   | A     |
| 76543 | CS-319    | A     |
| 76543 | CS-101    | A     |
| 00128 | CS-101    | A     |
| 12345 | CS-190    | A     |
| 12345 | CS-315    | A     |
| 54321 | CS-101    | A-    |
| 55739 | MU-199    | A-    |
| 45678 | CS-319    | B     |
| 19991 | HIS-351   | B     |
| 98765 | CS-315    | B     |
| 45678 | CS-101    | B+    |
| 54321 | CS-190    | B+    |
| 44553 | PHY-101   | B-    |
| 12345 | CS-101    | C     |
| 76653 | EE-181    | C     |
| 23121 | FIN-201   | C+    |
| 98765 | CS-101    | C-    |
| 45678 | CS-101    | F     |
+-------+-----------+-------+

Here is where grade is. There are 4 NULL values and 1 F.

mysql> select course_id, credits
    -> from course;
+-----------+---------+
| course_id | credits |
+-----------+---------+
| BIO-101   |       4 |
| BIO-301   |       4 |
| CS-101    |       4 |
| CS-190    |       4 |
| CS-315    |       3 |
| CS-319    |       3 |
| CS-347    |       3 |
| EE-181    |       3 |
| FIN-201   |       3 |
| HIS-351   |       3 |
| MU-199    |       3 |
| PHY-101   |       4 |
+-----------+---------+

This is where I get the number of credits from.

Just realized that 3 of the NULL values are from a previous question in the lab. Those entries are not represented in the course relation. My problem is solved. I am very sorry for all of the wasted time.

Answer 1

Try to change it to this :

UPDATE student as s
INNER JOIN takes as t ON s.ID = t.ID 
INNER JOIN course as c ON t.course_id = c.course_id
SET s.tot_cred = s.tot_cred - COALESCE(c.credits, 0)
WHERE t.grade = 'F'
OR
t.grade IS NULL

Answer 2

Check if you want to replace with INNER JOIN instead of LEFT JOIN as you might want to update only those records which as matching records.

UPDATE student s
LEFT JOIN takes as t ON s.ID = t.ID and t.grade = 'F' OR t.grade is NULL
LEFT JOIN course as c ON t.course_id = c.course_id
SET tot_cred = tot_cred - COALESCE(c.credits, 0);

Check the fiddle here

Answer 3

I did not realize that 3 of the NULL values were not represented in the course relation. This query works just fine, sorry for all the wasted time.