SQL if else in proc isn't working properly - beginner friendly - postgresql

Question

i'm pretty new to SQL and struggling with the last piece on my final project. Logically, I know how to do it (i think) I just can't get the syntax correct.
I have a teaches table and an instructor table. The teaches tables tells you what instructor has taught what class, when and for how many credits. The instructor table is obviously a table with IDs and instructor names. Finally, I have a table instructor_taught which really serves no purpose other than for us to learn. this table holds instructor's IDs and how many total credits they have taught.
My challenge is to create a proc that takes an instructor's ID and adds or updates their total credits taught in the instructor_taught table.
First: check if the ID already exists in the instructor_taught table...if it does not exist then I need to add it to the table. If it does exist then I need to update the entry and add the additional 3 credits. so if instructor 101 is already in the table with 3 total credits then I need to update the 3 to 6 instead of creating a new row.
I was able to create a proc that adds the instructor to the instructor_taught table but the update part is failing...so if i enter id:101 10x it will add that instructor 10x instead of updating.
Here is my code:
CREATE OR REPLACE PROCEDURE Day_21_monthlyPayment (IN id VARCHAR(30), INOUT d_count INTEGER DEFAULT 0)
LANGUAGE plpgsql
AS
$$
BEGIN
BEGIN
SELECT teaches.id, instructor.name, COUNT(*) AS d_count
FROM teaches NATURAL JOIN instructor
WHERE teaches.id = Day_21_monthlyPayment.id
GROUP BY teaches.id, instructor.name;
END
IF EXISTS(SELECT teaches.id FROM teaches)
THEN UPDATE instructor_course_nums SET tot_courses = d_count
WHERE teaches.id = Day_21_monthlyPayment.id
ELSE INSERT INTO instructor_course_nums (id, name, tot_courses)
END IF;
END;
$$
I"m pretty sure this is a simple if else statement but i can't get the syntax right. Thanks in advance!
DDL:
CREATE TABLE instructor(
ID VARCHAR(12),
name VARCHAR(30),
section VARCHAR(30),
salary NUMERIC(20)
)
CREATE TABLE teaches(
ID VARCHAR(12),
course VARCHAR(30),
count VARCHAR(30),
term VARCHAR(20),
year NUMERIC(20)
)
CREATE TABLE instructor_course_nums(
ID VARCHAR(12),
name VARCHAR(30),
tot_courses NUMERIC(2)
)

Answer 1

Even if you are forced to do a procedure, I think you can still do this in (nearly) one fell swoop with a single DML statement. Assuming you have the proper constraints (primary key), I would think something like this would work:
CREATE OR REPLACE PROCEDURE Day_21_monthlyPayment (idx VARCHAR(30), INOUT d_count INTEGER DEFAULT 0)
LANGUAGE plpgsql
AS
$BODY$
BEGIN
SELECT COUNT(*)
into d_count
FROM teaches t JOIN instructor i on t.id = i.id
WHERE t.id = idx;
insert into instructor_course_nums
select t.id, t.name, d_count
from instructor t
where t.id = idx
on conflict (id) do
update
set tot_courses = d_count;
END;
$BODY$