MySQL5.7中存储过程使用游标嵌套

MySQL 5.7中存储过程使用游标嵌套的语法如下：

DELIMITER $$
CREATE PROCEDURE sample_procedure()
BEGIN
    DECLARE done INT DEFAULT FALSE;
    DECLARE var1 INT;
    DECLARE var2 VARCHAR(255);
    DECLARE cur1 CURSOR FOR SELECT id, name FROM table1 WHERE condition;
    DECLARE cur2 CURSOR FOR SELECT id, name FROM table2 WHERE condition;
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;

    OPEN cur1;
    read_loop: LOOP
        FETCH cur1 INTO var1, var2;
        IF done THEN
            LEAVE read_loop;
        END IF;

        OPEN cur2;
        inner_read_loop: LOOP
            FETCH cur2 INTO var1, var2;
            IF done THEN
                LEAVE inner_read_loop;
            END IF;

            -- Do something with var1 and var2

        END LOOP;
        CLOSE cur2;

    END LOOP;
    CLOSE cur1;
END$$
DELIMITER ;

这个存储过程使用了两个游标`cur1`和`cur2`，`cur1`用于从`table1`中读取数据，`cur2`用于在每次循环中从`table2`中读取数据。在每次循环中，`FETCH`语句将从游标中读取一行数据，并将其存储在变量`var1`和`var2`中，然后可以在`-- Do something with var1 and var2`处对这些变量进行处理。