CREATE TRIGGER check_score BEFORE INSERT ON student FOR EACH ROW BEGIN IF NEW.score >= 60 THEN SET @sql = CONCAT('ALTER TABLE student MODIFY COLUMN score INT CHECK (score >= 60)'); PREPARE stmt FROM @sql; EXECUTE stmt; DEALLOCATE PREPARE stmt; END IF; END;这个里面的stmt是什么

在这个SQL语句中，stmt是一个用户定义的变量，用于存储动态生成的SQL语句。在这个例子中，stmt被用于存储动态生成的ALTER TABLE语句，该语句用于修改student表的score列，增加一个CHECK约束，以确保score列的值不小于60。

PREPARE语句用于准备动态生成的SQL语句，并将其存储在stmt变量中。EXECUTE语句用于执行stmt变量中的动态SQL语句。最后，DEALLOCATE PREPARE语句用于释放stmt变量占用的内存空间。

相关问题

CREATE TRIGGER trigger_price before UPDATE ON channel for each row BEGIN declare msg varchar(200); declare msh varchar(200); IF (NEW.price < 0 OR NEW.price > 100) THEN SET msg=CONCAT('不行'，NEW.price,'wo'); else SET msh= CONCAT('ok'); SIGNAL SQLSTATE 'HY000' SET MESSAGE_TEXT = msg; SIGNAL SQLSTATE 'HY000' SET MESSAGE_TEXT = msh; end if; END;错在哪

这个 TRIGGER 的问题在于在 IF 语句块外面定义了两个变量 msg 和 msh，但是在 IF 语句块内部没有使用它们。而在 IF 语句块内部定义的 msg 变量并没有被初始化，仅仅是定义了它的类型，这会导致在触发器中使用 SIGNAL 语句时出现错误。

下面是修改后的代码：

CREATE TRIGGER trigger_price BEFORE UPDATE ON channel 
FOR EACH ROW 
BEGIN     
    DECLARE msg VARCHAR(200); 		
    IF (NEW.price < 0 OR NEW.price > 100) THEN     
        SET msg = CONCAT('价格不能小于0或大于100，当前价格为', NEW.price); 		
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = msg;    
    END IF; 	
END;

修改后的代码中，只定义了一个 msg 变量，并且在 IF 语句块内部对它进行了初始化和赋值。如果触发器中的条件不满足，则会抛出一个信号，其中包含了错误信息。同时，将 SQLSTATE 设置为 '45000'，表示触发器抛出的是一个用户定义的错误。

create trigger _before_insert_orders before insert on orders for each row begin declare orders goods id int; declare goods_inventory_number decimal(10,2); declare orders_goods_name varchar(50); declare msg varchar (100); if not exists (select goods name from goods where goods id=new.goods id) then setmsg-’你订购的商品不存在！请重新下订单！！ signal sqlstate '45000' set message_text=msg; end if; select inventory_number,goods_name into goods_inventory_number,orders_goods_name from goods where goods_id=new.goods_id; if goods inventory number<new.number then setmsg=concat(orders_goods_name’库存不足，请修改订购商品的数量！”; signal sqlstate '45000' set message text=msg; else update goods set inventory number=inventory number-new.number where goods id=new.goods id; end if；end 检查这段代码哪里有错

这段代码有以下几个错误：

1. `declare orders goods id int;` 这行代码有语法错误，应该是 `declare orders_goods_id int;`。

2. `setmsg-’你订购的商品不存在！请重新下订单！！ signal sqlstate '45000' set message_text=msg;` 这行代码也有语法错误，`setmsg` 应该改为 `set msg`。

3. `select inventory_number,goods_name into goods_inventory_number,orders_goods_name from goods where goods_id=new.goods_id;` 这行代码应该是 `select inventory_number,goods_name into goods_inventory_number,orders_goods_name from goods where goods_id=new.goods_id;`。

4. `setmsg=concat(orders_goods_name’库存不足，请修改订购商品的数量！”; signal sqlstate '45000' set message text=msg;` 这行代码有语法错误，应该将 `orders_goods_name` 改为 `orders_goods_name + '`，并且将 `message text` 改为 `message_text`。

修改后的代码如下：

create trigger _before_insert_orders before insert on orders for each row
begin
    declare orders_goods_id int;
    declare goods_inventory_number decimal(10,2);
    declare orders_goods_name varchar(50);
    declare msg varchar (100);
    
    if not exists (select goods_name from goods where goods_id=new.goods_id) then
        set msg='你订购的商品不存在！请重新下订单！！';
        signal sqlstate '45000' set message_text=msg;
    end if;
    
    select inventory_number,goods_name into goods_inventory_number,orders_goods_name from goods where goods_id=new.goods_id;
    
    if goods_inventory_number<new.number then
        set msg=concat(orders_goods_name, '库存不足，请修改订购商品的数量！');
        signal sqlstate '45000' set message_text=msg;
    else
        update goods set inventory_number=inventory_number-new.number where goods_id=new.goods_id;
    end if;
end;