1 Joining Tables in Queries

2 Objectives You will be able to Write SQL queries that use Join operations to retrieve information from multiple tables.

3 Retrieving from Multiple Tables We often need to retrieve information from multiple tables in a single query. Example: Show the name of the customer contact for all orders in the Northwinds database.

The Northwinds Database (Subset)

5 Retrieving from Multiple Tables The Orders table has the Customer ID but not the customer’s contact name. The Customers table has the contact name. In order to determine the customer contact for an order, we need to get the Customer ID from the Orders table and then get the contact name for that Customer ID from the Customers table. The SQL “join” operation makes this possible with a single query. Retrieve pairs of rows from two tables having identical values of some key.

6 Types of Joins SQL defines several types of joins. The simplest is the inner join. It has the form: SELECT TABLE1.COLUMN1, TABLE2.COLUMN2 FROM TABLE1, TABLE2 WHERE TABLE1.COLUMN_X = TABLE2.COLUMN_Y Whatever you want from either table Query will return selected items from every pair of rows for which this condition is true.

7 Inner Join Example Running sqlcmd with the Northwind Database

8 Inner Join Example Objective: Determine the name of the customer's contact on all orders.

9 Inner Join Example 1> SELECT ORDERS.ORDERID, CUSTOMERS.CONTACTNAME 2> FROM ORDERS, CUSTOMERS 3> WHERE CUSTOMERS.CUSTOMERID = ORDERS.CUSTOMERID 4> GO ORDERID CONTACTNAME Maria Anders Maria Anders Maria Anders Maria Anders Maria Anders Maria Anders Carine Schmitt Carine Schmitt (830 rows affected) 1>

10 Being More Selective We could add more conditions to the WHERE clause. Suppose we wanted the contact name for just one specific order. We could add to the query: AND ORDERS.ORDERID=11012

11 Inner Join Example 1> SELECT ORDERS.ORDERID, CUSTOMERS.CONTACTNAME 2> FROM ORDERS, CUSTOMERS 3> WHERE CUSTOMERS.CUSTOMERID = ORDERS.CUSTOMERID 4> AND ORDERS.ORDERID= > GO ORDERID CONTACTNAME Peter Franken (1 rows affected) 1>

12 Table Aliases We can give a table a temporary name within a single SQL statement A short name saves keystokes and makes the statement easier to read and understand. Write the (short) temporary name immediately after the real table name in the FROM clause.

13 1> SELECT O.ORDERID, C.CONTACTNAME 2> FROM ORDERS O, CUSTOMERS C 3> WHERE C.CUSTOMERID = O.CUSTOMERID 4> AND O.ORDERID = > GO ORDERID CONTACTNAME Bernardo Batista (1 rows affected) 1> Table Aliases

14 Joining More than Two Tables Any number of tables can be included in a join operation. Suppose we want to see the name of each product on any order by Customer VINET VINET is the Customer ID

Joining More than Two Tables 1> SELECT P.ProductName 2> FROM Products P, [Order Details] OD, Orders O, Customers C 3> WHERE P.ProductID = OD.ProductID 4> AND OD.OrderID = O.OrderID 5> AND O.CustomerID = C.CustomerID 6> AND C.CustomerID = 'VINET' 7> GO ProductName Queso Cabrales Singaporean Hokkien Fried Mee Mozzarella di Giovanni Flotemysost Mozzarella di Giovanni Gnocchi di nonna Alice Konbu Jack's New England Clam Chowder Inlagd Sill Filo Mix (10 rows affected) 1>

17 The Northwind Product Browser Let’s extend the Northwind Product Browser to show the supplier for each product. Download the final version of the example done in class: Downloads/2011_04_05_Northwind_Product_Browser/ Downloads/2011_04_05_Northwind_Product_Browser/ File Northwind_Product_Browser.zip Expand zip file. Build and run.

18 The Northwinds Product Browser

19 The Northwind Product Browser We would like to show the supplier for each product along with the other product information. We can use a join to get related information along with the product information.

Products and Suppliers

21 Products and Suppliers The Product table includes a SupplierID. Not meaningful to the user. The Supplier table includes the supplier's CompanyName which is meaningful. Tables are related by SupplierID Use an inner join to get the CompanyName of the supplier of each product from the Supplier table.

22 Check Command in SQLCMD

23 Class Products Class Products is responsible for the query. SqlCommand1.CommandText = "SELECT P.*, S.CompanyName " + "FROM Products P, Suppliers S " + "WHERE P.SupplierID = S.SupplierID"; Build and run. Existing code is not affected by the change to the query.

24 Class Product public class Product { private String product_name; private int product_id; private decimal unit_price; private short units_in_stock; private bool discontinued; private String supplier_name;... public String Supplier_name { get { return supplier_name; } set { supplier_name = value; } }

25 Class Product // This constructor initializes a product // using a query result. public Product(SqlDataReader rdr) { Product_name = rdr.GetString(1); Product_id = rdr.GetInt32(0); Supplier_name = rdr.GetString(10); …

Class Product_Information

27 Product_Information.cs private void Display_Current_Record() { Product p = Product_List[current_record_number]; tbProductName.Text = p.Product_name; tbProductID.Text = p.Product_id.ToString(); tbUnitPrice.Text = p.Unit_price.ToString(); tbUnitsInStock.Text = p.Units_in_stock.ToString(); cbDiscontinued.Checked = p.Discontinued; tbSupplier.Text = p.Supplier_name; tbRecordNumber.Text = "Record " + (current_record_number + 1) + " of " + Product_List.Count; Update_Buttons(); }

28 Product Browser in Action

29 List Alphabetically Let’s show the products in alphabetical order by ProductName. Revised query: sc.CommandText = "SELECT P.*, S.CompanyName " + "FROM Products P, Suppliers S " + "WHERE P.SupplierID = S.SupplierID " + "ORDER BY P.ProductName" ;

30 App in Action End of Presentation