mysql> DELIMITER $
mysql> CREATE FUNCTION user_defined_var_demo()
-> RETURNS INT
-> BEGIN
-> SET @abc = 10;
-> return @abc;
-> END $
Query OK, 0 rows affected (0.03 sec)
mysql> DELIMITER ;
mysql>
mysql> SELECT @abc;
+------------+
| @abc |
+------------+
| NULL |
+------------+
1 row in set (0.00 sec)
mysql> SELECT user_defined_var_demo();
+-------------------------+
| user_defined_var_demo() |
+-------------------------+
| 10 |
+-------------------------+
1 row in set (0.02 sec)
mysql> SELECT @abc;
+------+
| @abc |
+------+
| 10 |
+------+
1 row in set (0.00 sec)