MySQL数据库循环表

Question

你能告诉我我的代码有什么问题吗？我跟着MySQL loop through tables循环了一个数据库的表。但是我没有从迭代光标中得到任何东西。 SELECT @sql;输出NULL值。谢谢！

DROP PROCEDURE IF EXISTS seedinvest.insertDatabaseRowHash;
DELIMITER $$
CREATE PROCEDURE seedinvest.insertDatabaseRowHash(_schemaName varchar(100))
BEGIN
    DECLARE done INT DEFAULT FALSE;
    DECLARE table_name varchar(20);
    DECLARE table_cursor cursor for SELECT table_name FROM information_schema.tables
                                    WHERE table_schema = _schemaName;
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;

    OPEN table_cursor;

    table_loop: LOOP
        FETCH table_cursor INTO table_name;
        IF done THEN
            LEAVE table_loop;
        END IF;

        SET @sql = CONCAT('DESC ', _schemaName, '.', table_name);
        SELECT @sql; -- It outputs Null value
        PREPARE stmt FROM @sql;
        EXECUTE stmt;
        DROP PREPARE stmt;
    END LOOP;

    CLOSE table_cursor;

END$$


Error Code: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'NULL' at line 1