SQL 查询以显示已购买所有 DVD 的客户的姓名

Question

这些是我创建的表并相应地插入了值：

CREATE TABLE Customer
(Customer_No INTEGER IDENTITY (1,1) PRIMARY KEY,
 Customer_Name VARCHAR(30) NOT NULL
)

CREATE TABLE DVD
(DVD_No INTEGER IDENTITY (1,1) PRIMARY KEY,
 DVD_Name VARCHAR(30)
)

CREATE TABLE DVD_Purchase
(DVD_Purchase_No INTEGER IDENTITY (1,1) PRIMARY KEY,
 DVD_No INTEGER NOT NULL,
 Customer_No INTEGER NOT NULL
 )

INSERT INTO Customer (Customer_Name)
VALUES('Daman')
INSERT INTO Customer (Customer_Name)
VALUES('Saif')
INSERT INTO Customer (Customer_Name)
VALUES('Gurung')
INSERT INTO Customer (Customer_Name)
VALUES('Upendra')

INSERT INTO DVD (DVD_Name)
VALUES('Bleach')
INSERT INTO DVD (DVD_Name)
VALUES('Gintama')
INSERT INTO DVD (DVD_Name)
VALUES('Tokyo Ghoul')
INSERT INTO DVD (DVD_Name)
VALUES('Death Note')

INSERT INTO DVD_Purchase (DVD_No,Customer_No)
VALUES (4,1)
INSERT INTO DVD_Purchase (DVD_No,Customer_No)
VALUES (1,2)
INSERT INTO DVD_Purchase (DVD_No,Customer_No)
VALUES (1,3)
INSERT INTO DVD_Purchase (DVD_No,Customer_No)
VALUES (2,3)
INSERT INTO DVD_Purchase (DVD_No,Customer_No)
VALUES (3,3)
INSERT INTO DVD_Purchase (DVD_No,Customer_No)
VALUES (4,3)
INSERT INTO DVD_Purchase (DVD_No,Customer_No)
VALUES (1,4)
INSERT INTO DVD_Purchase (DVD_No,Customer_No)
VALUES (2,4)

为了显示购买了所有 DVD 的客户的姓名，这是我尝试过的查询：

SELECT Customer_Name
FROM Customer
WHERE 
EXISTS (SELECT Customer_Name,DVD_Name
FROM Customer,DVD,DVD_Purchase
WHERE Customer.Customer_No = DVD_Purchase.Customer_No AND DVD.DVD_No = DVD_Purchase.DVD_No)

但不幸的是，在执行此查询后，它会显示所有客户姓名。但我想显示购买了所有 DVD 的特定客户名称。

有人可以为我提供正确的查询吗？

Answer 1

你可以尝试这样的事情：

select c.Customer_Name
from Customer c
join (  select Customer_No
        from DVD_Purchase
        group by Customer_No
        having count(distinct DVD_No) = (select count(*) from DVD)
        ) d on c.Customer_No = d.Customer_No

说明：首先我们需要知道可用的 DVD 数量。

select count(*) 
from DVD

然后我们需要获取客户购买的不同DVD的数量，并将其与我们之前获得的总数进行比较。

select Customer_No
from DVD_Purchase
group by Customer_No
having count(distinct DVD_No) = <number of DVDs available>

最后，我们只需要连接客户表来检索姓名。