Question

运行SQL Server 2012

我有一个查询，用于统计已签出图书的客户的数量。它还会计算那些两次签出同一本书的人。所有这些都被添加到专栏Customers Checked Out.中。问题是我不希望它计算那些已经多次检查过这本书的人。

例如，

Book Title                  Customers Checked OUt 
Title of Book1                         2
Title of Book2                         3
TItle of Book3                         1

在这种情况下，书籍1的标题实际上只签出了一个，但签出了两次。我怎么能只说1次而不是2次

添加查询

SELECT 
    Book.title AS 'Title of Book', 
    COUNT(Book_Rental.Book_id) AS 'Customers Checked Out'
FROM Book
JOIN Book_Rental ON Book.Book_id = Book_Rental.Book_id
JOIN Customer ON Book_Rental.Customer_id = Customer.customer_id
GROUP BY Book.Title
ORDER BY [# of Customers Rented] DESC;

Answer 1

据推测，您的查询类似于：

select BookTitle, count(*)
from CheckOuts
group by BookTitle

您想将count()更改为count(distinct)：

select BookTitle, count(distinct customerId)
from CheckOuts
group by BookTitle

由于您提供了查询，我认为这就是您想要的：

SELECT Book.title AS 'Title of Book', 
       COUNT(distinct Book_Rental.Customer_id) AS 'Customers Checked Out'
FROM Book
JOIN Book_Rental ON Book.Book_id = Book_Rental.Book_id
JOIN Customer ON Book_Rental.Customer_id = Customer.customer_id
GROUP BY Book.Title
ORDER BY [# of Customers Rented] DESC;

排除重复检出 - SQL

1 个答案: