A join links the rows in two or more tables by comparing the values in key columns and returning rows that have matching values. For example, this query joins the SalesOrderItems, Products, Employees, and SalesOrders tables to calculate sales totals and revenue:
SELECT Products.ID AS 'Product Code', Products.Name AS Item, Products.Description AS Style, SUM(SalesOrderItems.Quantity) AS Sales, Products.UnitPrice, SUM(SalesOrderItems.Quantity * Products.UnitPrice) AS Revenue FROM Employees JOIN SalesOrders ON SalesOrders.SalesRepresentative = Employees.EmployeeID JOIN SalesOrderItems ON SalesOrderItems.ID = SalesOrders.ID JOIN Products ON Products.ID = SalesOrderItems.ProductID GROUP BY Products.ID, Products.Description, Products.Name, Products.UnitPrice ORDER BY Sales