如果使用 SQL Server 的表中不存在值，有没有办法返回 0

Question

我在下面写了一个查询以从四个表中检索

Sales_Invoice, New_Customer, Company_Information, Expense

查询：

select
    isnull(sum(Expense.Expense_Amount), 0.00), as ExpenseAmount, 
    Company_Information.Company_Name,
    Sales_Invoice.Invoice_No, Sales_Invoice.Invoice_Date, Sales_Invoice.Item_Name,
    New_Customer.Customer_Name, New_Customer.Customer_ID
from 
    Sales_invoice, Company_Information, New_Customer, Expense
where 
    Sales_Invoice.Customer_Id = New_Customer.Customer_ID 
    and Sales_Invoice.Invoice_No = Expense.Invoice_No
group by
    Company_Information.Company_Name,
    Sales_Invoice.Invoice_No, Sales_Invoice.Invoice_Date, Sales_Invoice.Customer_ID, 
    Sales_Invoice.Item_Name, New_Customer.Customer_Name, New_Customer.Customer_ID 

查询运行良好，但如果Expense 表没有值Expense.Invoice_No 与Sales_Invoice.Invoice_No 不匹配，则上面的查询将返回空行。

但我想做的是，如果Expense.Invoice_No 不存在，那么我仍然希望我的行提供该费用金额返回 0.00

Answer 1

使用标准连接！然后，您可以使用left join 轻松处理“缺失”关系。

你的问题提示：

select
    coalesce(sum(e.expense_amount), 0.00) as expenseamount, 
    ci.company_name,
    si.invoice_no, si.invoice_date, si.item_name,
    nc.customer_name, nc.customer_id
from new_customer nc
inner join company_information ci on ???
inner join sales_invoice si       on si.customer_id = nc.customer_id 
left join expense e               on e.invoice_no = si.invoice_no
group by
    ci.company_name,
    si.invoice_no, si.invoice_date, si.customer_id, 
    si.item_name, nc.customer_name, nc.customer_id 

请注意，您的原始代码在我看来缺少客户和公司之间的连接条件。我在查询中将其表示为???。

您也可以使用相关子查询来表达相同的逻辑，这将避免外部聚合：

select
    (
        select coalesce(sum(e.expense_amount), 0.00) 
        from expense e 
        where e.invoice_no = si.invoice_no
    ) as expenseamount, 
    ci.company_name,
    si.invoice_no, si.invoice_date, si.item_name,
    nc.customer_name, nc.customer_id
from new_customer nc
inner join company_information ci on ???
inner join sales_invoice si on si.customer_id = nc.customer_id