MySQL IF ELSE 语句如何在存储过程中使用？

MySQL IF ELSE 语句在表达式计算结果为 false 时实现基本条件构造。其语法如下 –

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

语句必须以分号结尾。

为了演示在 MySQL 存储过程中使用 IF ELSE 语句，我们创建了以下存储过程：基于名为“student_info”的表的值，如下所示 –

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)

以下查询将创建一个名为“coursedetails_IFELSE”的过程，其中包含 IF ELSE 语句 –

mysql> DELIMITER // ;
mysql> CREATE PROCEDURE coursedetails_IFELSE(IN S_Subject Varchar(20), OUT S_Course varchar(50))
-> 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)';
-> ELSE
-> SET S_Course = 'Subject Not in the table ';
-> END IF;
-> END //
Query OK, 0 rows affected (0.00 sec)

现在，当我们调用这个过程时，我们可以看到下面的结果 –

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)