For oracle function to know if data is prime or not. Getting Warning: Function created with compilation errors - oracle

Question

Wanted to create a function that can return records containing the number data type which are prime number
But getting warning of compilation error. What is the mistake in the code.I am a beginner in pl/sql.
CREATE OR REPLACE FUNCTION isPrime (num number)
RETURN number
IS
retVal number;
BEGIN
DECLARE
prime_or_notPrime number;
counter number;
retVal:= 1;
prime_or_notPrime:= 1
counter:= 2
WHILE (counter <= num/2) LOOP
IF (mod(num ,counter)= 0) THEN
prime_or_notPrime: = 0
EXIT;
END IF;
IF (prime_or_notPrime = 1 ) THEN
retVal: = 1;
counter: = counter + 1
END IF;
END LOOP;
return retVal;
END;
/

Answer 1

What is the mistake in the code
The DECLARE is invalid syntax for a function/procedure. You want to declare the variables between the IS and BEGIN keywords.
Then you are missing a ; statement terminator after prime_or_notPrime:= 1 and counter:= 2.
Then you have:
prime_or_notPrime: = 0 instead of prime_or_notPrime := 0;
retVal: = 1; instead of retVal := 1; and
counter: = counter + 1 instead of counter := counter + 1; (they all have a space between : and = and some are missing ; again).
You never set retVal to anything other than 1.
You do not handle NULL values or values lower than 2.
Fixing that (and the indentation) gives:
CREATE OR REPLACE FUNCTION isPrime (num number)
RETURN number
IS
retVal number;
prime_or_notPrime number;
counter number;
BEGIN
IF NUM IS NULL OR NUM < 2 THEN
RETURN 0;
END IF;
retVal:= 1;
prime_or_notPrime:= 1;
counter:= 2;
WHILE (counter <= num/2) LOOP
IF (mod(num ,counter)= 0) THEN
prime_or_notPrime := 0;
retVal := 0;
EXIT;
END IF;
IF (prime_or_notPrime = 1 ) THEN
counter := counter + 1;
END IF;
END LOOP;
return retVal;
END;
/
Note: the prime_or_notprime variable is not controlling anything as, as soon as you set it to 0 you EXIT from the loop so it will never be used again. Having noted that, you can get rid of the final IF statement and just always increment the counter.
You can simplify it to:
CREATE OR REPLACE FUNCTION isPrime (
num number
) RETURN number DETERMINISTIC
IS
BEGIN
IF NUM IS NULL OR NUM < 2 THEN
RETURN 0;
END IF;
FOR counter IN 2 .. SQRT(num) LOOP
IF MOD(num, counter) = 0 THEN
RETURN 0;
END IF;
END LOOP;
RETURN 1;
END;
/
You can then consider improvements. Such as starting by checking 2 as a special case and then skipping all the other even numbers.
db<>fiddle here