why we can use = and := both with set? - mysql

Question

Generally we know that '=' operator using for comparison and ':=' for assignment,
But while using with SET both are working as assignment operator why?
below stored procedure
for example:
DELIMITER $$
CREATE DEFINER=`root`#`localhost` PROCEDURE `substringExample`()
BEGIN
DECLARE x varchar(7);
DECLARE num int;
DECLARE inc int;
SET inc:= 1;
WHILE inc<1400 DO
SELECT SUBSTRING(USER_TEMP_NUM, 8, 13) AS ExtractString
INTO x FROM USER_REGISTRATION_DETAILS where sl_no=inc;
SET num= CONVERT(x,int);
IF (num%2=0) THEN
SELECT num;
END IF;
SET inc:=inc+1;
END WHILE;
END$$
DELIMITER ;
in the above code SET num= CONVERT(x,int); gives the correct output as well as SET num:= CONVERT(x,int);
I am beginner of stored procedure so dont know at expert level

Answer 1

From John Woo's answer:
Both of them are assignment operators but one thing I can find their differences is that = can be used to perform boolean operation while := cannot.
valid: SUM(val = 0)
Invalid: SUM(val := 0)
FROM User-Defined Variables
One more thing, You can also assign a value to a user variable in statements other than SET. In this case, the assignment operator must be := and not = because the latter is treated as the comparison operator = in non-SET statements.
mysql> SET #t1=1, #t2=2, #t3:=4;
mysql> SELECT #t1, #t2, #t3, #t4 := #t1+#t2+#t3;
+------+------+------+--------------------+
| #t1 | #t2 | #t3 | #t4 := #t1+#t2+#t3 |
+------+------+------+--------------------+
| 1 | 2 | 4 | 7 |
+------+------+------+--------------------+
And also check this link, hope it helps you,
What is the difference between := and = mysql assignment operator