sql - 编写查询以显示来自两个不同表的数据

Question

我试图将 Vendors 表中的一列作为“VendorName”，并将 Invoices 表中 InvoiceTotal 列中的一列作为 InvoiceAverage。我正在尝试平均表中每个供应商的发票总额

SELECT InvoiceTotal
FROM Invoices
GROUP BY Invoices.InvoiceTotal
ORDER BY AVG(InvoiceTotal)

据我所知，它没有给我任何我想要的东西

Answer 1

假设您加入字段 VendorId：

SELECT v.VendorName, AVG(i.InvoiceTotal) AS InvoiceAverage
FROM Invoices i
    JOIN Vendors v ON i.VendorId = v.VendorId
GROUP BY v.VendorName
ORDER BY AVG(i.InvoiceTotal)

Answer 2

select VendorName, avg(InvoiceTotal) as 'some column name'
from Invoices
group by VendorName
-- order by avg(InvoiceTotal) desc/ asc

您想按平均的值进行分组。这就是它的工作原理。