MySQL IF 语句如何在存储过程中使用？

IF expression THEN
Statements;
END IF;

mysql> Select * from student_info;
+------+---------+------------+------------+
| id   | Name    | Address    | Subject    |
+------+---------+------------+------------+
| 101  | YashPal | Amritsar   | History    |
| 105  | Gaurav  | Jaipur     | Literature |
| 125  | Raman   | Shimla     | Computers  |
+------+---------+------------+------------+
3 rows in set (0.00 sec)

mysql> DELIMITER // ;
mysql> CREATE PROCEDURE coursedetails_IF(IN S_Subject Varchar(20), OUT S_Course varchar(20))
    -> BEGIN
    -> DECLARE Sub Varchar(20);
    -> SELECT Subject INTO SUB
    -> FROM Student_info WHERE S_Subject = Subject;
    -> IF Sub = 'Computers' THEN
    -> SET S_Course = 'B.Tech(CSE)';
    -> END IF;
    -> END //
Query OK, 0 rows affected (0.00 sec)

mysql> Delimiter ; //
mysql> CALL coursedetails_IF('Computers', @S_Course);
Query OK, 1 row affected (0.00 sec)

mysql> Select @S_Course;
+-------------+
| @S_Course   |
+-------------+
| B.Tech(CSE) |
+-------------+
1 row in set (0.00 sec)