Mysql存储过程使用游标返回null

Question

代码如下：当我调用noemail时返回null，这是怎么回事？

而且那里有一个空的cust_email（见表customers），但它什么都不返回，只是空，它应该返回一个cust_name，哪一部分错了？谢谢。

    DELIMITER $$
    USE a_schema_in_mysql $$
    create procedure noemail()
    begin
        declare cust_name char;
        declare custcursor cursor for
            select cust_name from customers where cust_email is null;

        open custcursor;
        loop
            fetch custcursor into cust_name;
            select concat('Null email host is',cust_name) as noemailcust;
        end loop;
        close custcursor;
    end$$
    DELIMITER ;

    select * from customers；

    +------------+---------------+----------------------+-----------+------------+
    | cust_id    | cust_name     | cust_address         | cust_city | cust_state |
    +------------+---------------+----------------------+-----------+------------+
    | 1000000001 | Village Toys  | 200 Maple Lane       | Detroit   | MI         |
    | 1000000002 | Kids Place    | 333 South Lake Drive | Columbus  | OH         | 
    | 1000000003 | Fun4All       | 1 Sunny Place        | Muncie    | IN         | 
    | 1000000004 | Fun4All       | 829 Riverside Drive  | Phoenix   | AZ         | 
    | 1000000005 | The Toy Store | 4545 53rd Street     | Chicago   | IL         |
    +------------+---------------+----------------------+-----------+------------+
    +----------+--------------+--------------------+-----------------------+
    | cust_zip | cust_country | cust_contact       | cust_email            |
    +----------+--------------+--------------------+-----------------------+
    | 44444    | USA          | John Smith         | sales@villagetoys.com |
    | 43333    | USA          | Michelle Green     | NULL                  |
    | 42222    | USA          | Jim Jones          | jjones@fun4all.com    |
    | 88888    | USA          | Denise L. Stephens | dstephens@fun4all.com |
    | 54545    | USA          | Kim Howard         | kim@thetoystore.com   |
    +----------+--------------+--------------------+-----------------------+

以及下面的错误：

mysql> call noemail;
+-------------+
| noemailcust |
+-------------+
| NULL        |
+-------------+
1 row in set (0.00 sec)

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

帮帮我，谢谢！

Answer 1

您使用的是同名“cust_name”，这也是您的列名。 当参数或变量名称与字段名称冲突时，使用参数或变量名称。

我正在添加编辑：

DELIMITER $$
USE a_schema_in_mysql $$
create procedure noemail()
begin
    declare cust_name_s char;
    declare custcursor cursor for
        select cust_name from customers where cust_email is null;

    open custcursor;
    loop
        fetch custcursor into cust_name_s;
        select concat('Null email host is',cust_name_s) as noemailcust;
    end loop;
    close custcursor;
end$$
DELIMITER ;

Answer 2

我假设你可以在不使用光标的情况下实现你想要的

DELIMITER $$
USE a_schema_in_mysql $$
create procedure noemail()
begin
    select concat('Null email host is',cust_name) from customers where cust_email is null;
end$$
DELIMITER ;