MySQL存储过程---游标

什么是游标

保存select语句的数据集，主要用于对数据集逐行进行处理。

游标的操作过程

1、定义游标

DECLARE cur_name CURSOR FOR SELECT COLUMN FROM TABLE WHERE CONDITION;

2、打开游标

open cur_name;

3、从游标提取数据

FETCH cur_name INTO var1,var2,...;

4、游标的异常处理

declare done int default 1;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 0;

5、关闭游标

close cur_name；

cursor应用举例

1、创建存储过程

DELIMITER $$

USE `world`$$

DROP PROCEDURE IF EXISTS `p_cur1`$$

CREATE DEFINER=`root`@`10.0.0.%` PROCEDURE `p_cur1`()
BEGIN
    
    DECLARE u_id,u_a INT;
    DECLARE u_n,u_g VARCHAR(64);
    
    DECLARE cur1 CURSOR FOR SELECT id,NAME,age,gender FROM t4 WHERE id <5;
    OPEN cur1;
    FETCH cur1 INTO u_id,u_n,u_a,u_g;
    SELECT u_id,u_n,u_a,u_g;
    FETCH cur1 INTO u_id,u_n,u_a,u_g;
    SELECT u_id,u_n,u_a,u_g;
    FETCH cur1 INTO u_id,u_n,u_a,u_g;
    SELECT u_id,u_n,u_a,u_g;
    FETCH cur1 INTO u_id,u_n,u_a,u_g;
    SELECT u_id,u_n,u_a,u_g;
    CLOSE cur1;
    
    END$$

DELIMITER ;

调用

mysql> call p_cur1();
+------+----------+------+------+
| u_id | u_n      | u_a  | u_g  |
+------+----------+------+------+
|    1 | a4182e_3 |   27 | F    |
+------+----------+------+------+
1 row in set (0.01 sec)

+------+----------+------+------+
| u_id | u_n      | u_a  | u_g  |
+------+----------+------+------+
|    2 | a4187f_5 |   33 | M    |
+------+----------+------+------+
1 row in set (0.01 sec)

+------+----------+------+------+
| u_id | u_n      | u_a  | u_g  |
+------+----------+------+------+
|    3 | a418ba_7 |   34 | M    |
+------+----------+------+------+
1 row in set (0.01 sec)

+------+----------+------+------+
| u_id | u_n      | u_a  | u_g  |
+------+----------+------+------+
|    4 | a418dc_9 |   30 | F    |
+------+----------+------+------+
1 row in set (0.01 sec)

Query OK, 0 rows affected (0.01 sec)

异常处理

当给定的条件不满足时，会抛出异常，比如上面的存储过程p_cur1，cursor获取的数据超过了表的行数，就会报错：

mysql> call p_cur1();
+------+----------+------+------+
| u_id | u_n      | u_a  | u_g  |
+------+----------+------+------+
|    1 | a4182e_3 |   27 | F    |
+------+----------+------+------+
1 row in set (0.00 sec)

+------+----------+------+------+
| u_id | u_n      | u_a  | u_g  |
+------+----------+------+------+
|    2 | a4187f_5 |   33 | M    |
+------+----------+------+------+
1 row in set (0.00 sec)

+------+----------+------+------+
| u_id | u_n      | u_a  | u_g  |
+------+----------+------+------+
|    3 | a418ba_7 |   34 | M    |
+------+----------+------+------+
1 row in set (0.00 sec)

+------+----------+------+------+
| u_id | u_n      | u_a  | u_g  |
+------+----------+------+------+
|    4 | a418dc_9 |   30 | F    |
+------+----------+------+------+
1 row in set (0.00 sec)

ERROR 1329 (02000): No data - zero rows fetched, selected, or processed

改写存储过程，添加异常处理

DELIMITER $$

USE `world`$$

DROP PROCEDURE IF EXISTS `p_cur1`$$

CREATE DEFINER=`root`@`10.0.0.%` PROCEDURE `p_cur1`()
BEGIN
    
    DECLARE u_id,u_a INT;
    DECLARE u_n,u_g VARCHAR(64);
    DECLARE done INT DEFAULT 1;
    
    DECLARE cur1 CURSOR FOR SELECT id,NAME,age,gender FROM t4 WHERE id <5;
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done=0;
    OPEN cur1;
    FETCH cur1 INTO u_id,u_n,u_a,u_g;      ## 处理重复最后一条数据的问题
    WHILE done=1
    DO
    SELECT u_id,u_n,u_a,u_g;
    FETCH cur1 INTO u_id,u_n,u_a,u_g;

    END WHILE;

    CLOSE cur1;
    
    END$$

DELIMITER ;

调用

mysql> call p_cur1();
+------+----------+------+------+
| u_id | u_n      | u_a  | u_g  |
+------+----------+------+------+
|    1 | a4182e_3 |   27 | F    |
+------+----------+------+------+
1 row in set (0.01 sec)

+------+----------+------+------+
| u_id | u_n      | u_a  | u_g  |
+------+----------+------+------+
|    2 | a4187f_5 |   33 | M    |
+------+----------+------+------+
1 row in set (0.01 sec)

+------+----------+------+------+
| u_id | u_n      | u_a  | u_g  |
+------+----------+------+------+
|    3 | a418ba_7 |   34 | M    |
+------+----------+------+------+
1 row in set (0.01 sec)

+------+----------+------+------+
| u_id | u_n      | u_a  | u_g  |
+------+----------+------+------+
|    4 | a418dc_9 |   30 | F    |
+------+----------+------+------+
1 row in set (0.01 sec)

Query OK, 0 rows affected (0.01 sec)