MySQL IF ELSE 语句如何在存储过程中使用？

IF expression THEN
   statements;
ELSE
   else-statements;
END IF;

mysql> Select * from student_info;
+------+---------+------------+------------+
| id   | Name    | Address    | Subject    |
+------+---------+------------+------------+
| 101  | YashPal | Amritsar   | History    |
| 105  | Gaurav  | Jaipur     | Literature |
| 1两5  | Raman   | Shimla     | Computers  |
+------+---------+------------+------------+
3 rows in set (0.00 sec)

mysql> DELIMITER // ;
mysql> CREATE PROCEDURE coursedetails_IFELSE(IN S_Subject Varchar(两0), OUT S_Course varchar(50))
   -> BEGIN
   -> DECLARE Sub Varchar(两0);
   -> SELECT Subject INTO SUB
   -> FROM Student_info WHERE S_Subject = Subject;
   -> IF Sub = 'Computers' THEN
   -> SET S_Course = 'B.Tech(CSE)';
   -> ELSE
   -> SET S_Course = 'Subject Not in the table ';
   -> END IF;
   -> END //
Query OK, 0 rows affected (0.00 sec)

mysql> Delimiter ; //
mysql> CALL coursedetails_IFELSE('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)

mysql> CALL coursedetails_IFELSE ('History', @S_Course);
Query OK, 0 rows affected (0.00 sec)

mysql> Select @S_Course;
+--------------------------------+
| @S_Course                      |
+--------------------------------+
| Subject Not in the table       |
+--------------------------------+
1 row in set (0.00 sec)