What happens when two "INNER JOIN" statements are used in one SQL query?
SELECT Products.ProductID, Products.ProductName, Products.SupplierID, Products.CategoryID, Products.QuantityPerUnit, Products.UnitPrice, Products.UnitsInStock, Products.UnitsOnOrder, Products.ReorderLevel, Categories.CategoryName, Suppliers.CompanyName FROM Products INNER JOINCategories ON Products.CategoryID = Categories.CategoryID INNER JOIN Suppliers ONProducts.SupplierID = Suppliers.SupplierID
Hi, I'm trying to understanding what is happening in the Northwind database query above. I understand the "SELECT" statement but everything from the "FROM" statement onwards is a bit confusing to me. I was hoping that someone could break it down for me.
Also, shouldn't the Suppliers.SupplierID and Categories.CategoeryID be in the SELECT statement? And Products has a "many to one" relationship with Suppliers and Categories, but what would happen with this statement if the relationship was "one to many"? Would I end up with duplicate Products?
Who is Participating?
Experts Exchange Solution brought to you by
"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.
Experts Exchange Solution brought to you by
Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.
Start your 7-day free trialIt's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
From novice to tech pro — start learning today.
Experts Exchange Solution brought to you by
So in the above example the first INNER JOIN is only pulling records where the CategoryIDs match between the Products and Categories tables. The second INNER JOIN is filtering the results further by including only the results where the SupplierIDs match between the Products an Suppliers tables.
No - you don't have to include the joining columns in the select statement when doing a join.
If the relationship were switched to a "one to many" (i.e. one product can have one or more categories and one or more suppliers) then the database structure would be a little different. There are a few ways to implement this, but one way would be to add a ProductID column on the Categories and Suppliers tables. Your statement would then look something like this...
SELECT *
FROM Products
INNER JOIN Categories
ON Categories.ProductID = Products.ProductID
INNER JOIN Suppliers
ON Suppliers.ProductID = Product.ProductID
You won't be able to have duplicate products because of the ProductID primary key constraint; however, there may appear to be duplicates in your result set because you are joining data from multiple tables.