MySQL 如何在存储过程中使用IF ELSE语句

MySQL的 IF ELSE 语句实现基本的条件构造，当表达式的值为false时。语法如下 −

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

语句必须以分号结束。

为了演示在MySQL存储过程中使用 IF ELSE 语句，我们创建以下存储过程，该存储过程基于名为“student_info”的表的以下值−

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”的存储过程，其中含有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 = '表中没有此科目';
  -> END IF;
  -> END //
Query OK, 0 rows affected (0.00 sec)

现在，当我们调用这个过程时，结果如下 −

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                    |
+--------------------------------+
| 表中没有此科目                |
+--------------------------------+
1 row in set (0.00 sec)

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