显然fetch返回NULL

Question

我正在学习存储过程,mysql中的游标,我偶然发现它:

delimiter //

CREATE PROCEDURE some_func()
BEGIN
    DECLARE link_rewrite VARCHAR(255);
    DECLARE link_rewrite_cursor CURSOR FOR SELECT link_rewrite FROM prod;

    OPEN link_rewrite_cursor;

    SET @count = 0;

    WHILE @count < 10 DO
        FETCH link_rewrite_cursor INTO link_rewrite;
        SELECT link_rewrite;
        set @count = @count + 1;
    END WHILE;

    CLOSE link_rewrite_cursor;

END//

delimiter ;

我的问题是:为什么SELECT link_rewrite总是返回NULL(在prod表中有9000行).SELECT link_rewrite FROM prod返回很多行(9000行).

Answer 1

您应避免对多个不同的事物使用相同的名称.具体来说,为变量指定与您选择的列不同的名称.例如,如果重命名变量v_link_rewrite,那么它可能会起作用:

delimiter //

DROP PROCEDURE IF EXISTS some_func //

CREATE PROCEDURE some_func()
BEGIN
    DECLARE v_link_rewrite VARCHAR(255);
    DECLARE link_rewrite_cursor CURSOR FOR SELECT link_rewrite FROM prod;

    OPEN link_rewrite_cursor;

    SET @count = 0;

    WHILE @count < 10 DO
        FETCH link_rewrite_cursor INTO v_link_rewrite;
        SELECT v_link_rewrite;
        set @count = @count + 1;
    END WHILE;

    CLOSE link_rewrite_cursor;

END//

delimiter ;

Answer 2

如果您只想选择前 10 行，请执行以下操作：

select link_rewrite from prod limit 10

它要快得多，而且您不必使用光标。