Question

我有一个小的声明，递减一个值：

UPDATE cart_items SET quantity = quantity - 1
WHERE cart_id = {$cart_id} AND id = {$cart_item_id}

但如果该值在减量后变为0，那么SQL是否可以删除该行？如果是这样，那么我想重新计算与该购物车匹配的行数：

SELECT FROM cart_items WHERE cart_id = {$cart_id}

如果行数为零，我想从另一个表中删除该记录，如下所示：

DELETE FROM cart WHERE id = {$cart_id}

目前似乎需要多个查询来执行此操作，但是这一切都可以在单个SQL语句中完成吗？

Answer 1

简短的回答是，如果不将额外的查询包含在触发器或过程中，则无法实现。

您可以在交易中执行此操作，但不需要SELECT，但需要3次查询：

START TRANSACTION;

    UPDATE cart_items 
       SET quantity = quantity - 1
     WHERE cart_id = {$cart_id} 
       AND id = {$cart_item_id};

    DELETE 
      FROM cart_items
     WHERE quantity = 0
       AND cart_id = {$cart_id} 
       AND id = {$cart_item_id};

    DELETE c
      FROM cart c
 LEFT JOIN cart_items ci
        ON ci.cart_id = c.id
     WHERE c.id = {$cart_id}
       AND ci.cart_id IS NULL;

COMMIT;

最后DELETE将购物车加入cart_items，如果找不到购物车，则会删除购物车（cart_items字段为NULL）。

我已经包含了可用的标识符以加快DELETE的速度，虽然它们没有它们应该没问题。它只会查找并选择任何其他数量0项或空车。

Answer 2

我认为即使您将使用触发器，也无法在一个查询中执行所有操作，因为您将收到错误：

CREATE TABLE cart_items (cart_id int key, quantity int);
INSERT INTO cart_items VALUES (1, 1), (2, 2);
-- Create trigger
delimiter |
CREATE TRIGGER update_cart_items AFTER UPDATE ON cart_items
  FOR EACH ROW
  BEGIN
    DELETE FROM cart_items WHERE quantity = 0;
  END;
|
delimiter ;

现在，如果你要运行更新查询：

UPDATE cart_items SET quantity = quantity - 1 WHERE cart_id = 1;

您将收到错误：

ERROR 1442 (HY000): Can't update table 'cart_items' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.

这就是我认为你应该使用少量查询的原因......

使用一个SQL查询进行SELECT，UPDATE，DELETE？

2 个答案: