实际上，CASE语句具有IF-THEN-ELSE语句的功能。它的语法如下：
CASEWHEN condition_1 THEN {...statements to execute when condition_1 is TRUE...}[ WHEN condition_2 THEN {...statements to execute when condition_2 is TRUE...} ][ WHEN condition_n THEN {...statements to execute when condition_n is TRUE...} ][ ELSE {...statements to execute when all conditions were FALSE...} ]END CASE;
CASE语句将在没有任何WHEN子句被执行时执行ELSE子句。
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为了演示在MySQL存储过程中使用CASE语句的用法，我们创建了以下基于表格‘student_info’的存储过程，其值如下所示：
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)
下面的查询将创建一个名为‘coursedetails_CASE’的存储过程，其中包含一个CASE语句−
mysql> Delimiter //mysql> CREATE PROCEDURE coursedetails_CASE(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; -> CASE S_Subject WHEN 'Computers' THEN -> SET S_Course = 'B.Tech(CSE)’; -> WHEN 'History' THEN -> SET S_Course = 'Masters in History'; -> WHEN 'Literature' THEN -> SET S_Course = 'Masters in English'; -> ELSE -> SET S_Course = 'Subject not in the table'; -> END CASE ; -> END //Query OK, 0 rows affected (0.11 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_CASE ('Computers', @S_course);Query OK, 1 row affected (0.08 sec)mysql> Select @S_Course;+-------------+| @S_Course |+-------------+| B.Tech(CSE) |+-------------+1 row in set (0.00 sec)mysql> CALL coursedetails_CASE ('literature', @S_course);Query OK, 1 row affected (0.00 sec)mysql> Select @S_Course;+--------------------+| @S_Course |+--------------------+| Masters in English |+--------------------+1 row in set (0.00 sec)mysql> CALL coursedetails_CASE ('Math', @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)mysql> CALL coursedetails_CASE ('History', @S_course);Query OK, 1 row affected (0.01 sec)mysql> Select @S_Course;+--------------------+| @S_Course |+--------------------+| Masters in History |+--------------------+1 row in set (0.00 sec)
以上就是MySQL CASE语句如何在存储过程中使用？的详细内容，更多请关注创想鸟其它相关文章！
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