MySQL：子查询：无输出

Question

我正在研究子查询。我有个问题。是什么导致什么都没有回来？我以为我已经让它变得非常流畅，但看起来我做错了什么......它是WHERE运算符吗？

意图：

我试图显示尚未订购的所有商品。每个项目都有一个包含在order_details中的item_ID。我必须使用子查询。

我没有收到任何错误，它只显示任何内容。建议？

查询：

SELECT
        (SELECT items.item_id 
        FROM items 
        ) as 'Item ID'
FROM items
JOIN order_details on order_details.item_ID = items.item_id
WHERE order_details.order_qty < 1
ORDER BY items.item_id ASC;

所有其他代码：

CREATE TABLE items
(
 item_id    INT       NOT NULL,
 title     VARCHAR(50)   NOT NULL,
 artist_id   INT   NOT NULL,
 unit_price  DECIMAL(9,2)  NOT NULL,
 CONSTRAINT items_pk 
  PRIMARY KEY (item_id),
 CONSTRAINT items_fk_artists
  FOREIGN KEY (artist_id) REFERENCES artists (artist_id)
);

CREATE TABLE orders
(
 order_id     INT     NOT NULL,
 customer_id    INT     NOT NULL,
 order_date    DATE    NOT NULL,
 shipped_date   DATE,
 employee_id    INT,
 CONSTRAINT orders_pk
  PRIMARY KEY (order_id),
 CONSTRAINT orders_fk_customers
  FOREIGN KEY (customer_id) REFERENCES customers (customer_id),
CONSTRAINT orders_fk_employees
  FOREIGN KEY (employee_id) REFERENCES employees (employee_id)
);

CREATE TABLE order_details
(
 order_id   INT      NOT NULL,
 item_id    INT      NOT NULL,
 order_qty   INT      NOT NULL,
 CONSTRAINT order_details_pk 
  PRIMARY KEY (order_id, item_id),
 CONSTRAINT order_details_fk_orders
  FOREIGN KEY (order_id)
  REFERENCES orders (order_id),
 CONSTRAINT order_details_fk_items
  FOREIGN KEY (item_id)
  REFERENCES items (item_id)
);


INSERT INTO items (item_id,title,artist_id,unit_price) VALUES 
(1,'Umami In Concert',10,17.95),
(2,'Race Car Sounds',11,13),
(3,'No Rest For The Weary',12,16.95),
(4,'More Songs About Structures and Comestibles',12,17.95),
(5,'On The Road With Burt Ruggles',13,17.5),
(6,'No Fixed Address',14,16.95),
(7,'Rude Noises',15,13),
(8,'Burt Ruggles: An Intimate Portrait',13,17.95),
(9,'Zone Out With Umami',10,16.95),
(10,'Etcetera',16,17);

INSERT INTO orders VALUES
(19,1,'2012-10-23','2012-10-28',6),
(29,8,'2012-11-05','2012-11-11',6),
(32,11,'2012-11-10','2012-11-13',NULL),
(45,2,'2012-11-25','2012-11-30',NULL),
(70,10,'2012-12-28','2013-01-07',5),
(89,22,'2013-01-20','2013-01-22',7),
(97,20,'2013-01-29','2013-02-02',5),
(118,3,'2013-02-24','2013-02-28',7),
(144,17,'2013-03-21','2013-03-29',NULL),
(158,9,'2013-04-04','2013-04-20',NULL),
(165,14,'2013-04-11','2013-04-13',NULL),
(180,24,'2013-04-25','2013-05-30',NULL),
(231,15,'2013-06-14','2013-06-22',NULL),
(242,23,'2013-06-24','2013-07-06',3),
(264,9,'2013-07-15','2013-07-18',6),
(298,18,'2013-08-18','2013-09-22',3),
(321,2,'2013-09-09','2013-10-05',6),
(381,7,'2013-11-08','2013-11-16',7),
(413,17,'2013-12-05','2014-01-11',7),
(442,5,'2013-12-28','2014-01-03',5),
(479,1,'2014-01-30','2014-03-03',3),
(491,16,'2014-02-08','2014-02-14',5),
(523,3,'2014-03-07','2014-03-15',3),
(548,2,'2014-03-22','2014-04-18',NULL),
(550,17,'2014-03-23','2014-04-03',NULL),
(601,16,'2014-04-21','2014-04-27',NULL),
(607,20,'2014-04-25','2014-05-04',NULL),
(624,2,'2014-05-04','2014-05-09',NULL),
(627,17,'2014-05-05','2014-05-10',NULL),
(630,20,'2014-05-08','2014-05-18',7),
(651,12,'2014-05-19','2014-06-02',7),
(658,12,'2014-05-23','2014-06-02',7),
(687,17,'2014-06-05','2014-06-08',NULL),
(693,9,'2014-06-07','2014-06-19',NULL),
(703,19,'2014-06-12','2014-06-19',7),
(778,13,'2014-07-12','2014-07-21',7),
(796,17,'2014-07-19','2014-07-26',5),
(800,19,'2014-07-21','2014-07-28',NULL),
(802,2,'2014-07-21','2014-07-31',NULL),
(824,1,'2014-08-01',NULL,NULL),
(827,18,'2014-08-02',NULL,NULL),
(829,9,'2014-08-02',NULL,NULL);

INSERT INTO order_details VALUES 
(381,1,1),
(601,9,1),
(442,1,1),
(523,9,1),
(630,5,1),
(778,1,1),
(693,10,1),
(118,1,1),
(264,7,1),
(607,10,1),
(624,7,1),
(658,1,1),
(800,5,1),
(158,3,1),
(321,10,1),
(687,6,1),
(827,6,1),
(144,3,1),
(479,1,2),
(630,6,2),
(796,5,1),
(97,4,1),
(601,5,1),
(800,1,1),
(29,10,1),
(70,1,1),
(165,4,1),
(180,4,1),
(231,10,1),
(413,10,1),
(491,6,1),
(607,3,1),
(651,3,1),
(703,4,1),
(802,3,1),
(824,7,2),
(829,1,1),
(550,4,1),
(796,7,1),
(693,6,1),
(29,3,1),
(32,7,1),
(242,1,1),
(298,1,1),
(479,4,1),
(548,9,1),
(627,9,1),
(778,3,1),
(19,5,1),
(89,4,1),
(242,6,1),
(264,4,1),
(550,1,1),
(693,7,3),
(824,3,1),
(829,5,1),
(829,9,1);

Answer 1

WHERE order_details.order_qty < 1 : 

这在您的数据中失败，其中order_details.order_qty＆lt; 1.因为它没有任何数量的记录＆lt; 1，所以它没有显示。

由于此子查询将返回多于1行，因此在这种情况下它将不起作用。要显示未使用子查询订购的所有项目：

Select distinct item_id from items where item_id not in (select distinct item_id from order_details);

在这里，我正在使用项目表中的item_id但不是订单详细信息。所以这是那些尚未订购的商品的清单。

Answer 2

您选择的order_qty小于1

WHERE order_details.order_qty < 1

以下是qty＆gt;的更新查询1：

SELECT items.item_id 
FROM items
inner JOIN order_details on order_details.item_ID = items.item_id
WHERE order_details.order_qty > 1
ORDER BY items.item_id ASC;

[编辑]

将此查询用于尚未订购的项目

select i.* from items i
left outer join order_details od on od.item_id = i.item_id
where order_qty is null