| Up to this point you have focused primarily on extracting data from a single table. Depending on how advanced your database becomes, at times you might want to extract data from multiple tables at once. If that is the case, you will need to use joins. Although there are several types of joins, two types will be covered here: Inner Joins Of the different types of joins, inner joins are by far the most popular. Inner joins allow you to see all the records of two tables that have a relation established for one another. Remember, the customers table and the creditcards table had a relationship established. The two tables are similar to the following: | CustomerID | FirstName | LastName | | 2 | Zak | Ruvalcaba | | 4 | Matthew | Pizzi | | CustomerID | CreditCardType | CreditCardExpiration | | 2 | Visa | 5555555555555555 | | 4 | MasterCard | 3333444455553333 | Assume that you wanted to extract the information from the customers table for CustomerID 2. Before inner joins, you would have to perform two Select statements: Select * From Customers Where CustomerID = 2 And Select * From CreditCards Where CustomerID = 2 You can begin to see how tedious this could get, not to mention that it is completely inefficient. To solve this problem, an inner joins could be performed: Select Customers.CustomerID, Customers.FirstName, Customers.LastName, CreditCards.CustomerID, CreditCards.CreditCardType, CreditCards.CreditCardExpiration From Customers Inner Join CreditCards On Customers.CustomerID = CreditCards.CustomerID The joins would effectively produce one virtual table with the following results: | CustomerID | FirstName | LastName | CreditCardType | CreditCardExpiration | | 2 | Zak | Ruvalcaba | Visa | 555555555555555 | | 4 | Matthew | Pizzi | MasterCard | 3333444455553333 | You will notice that the preceding table now becomes more efficient and manageable. Notice that rather than referencing the names of the tables, you used TableName.Field notation. This is crucial when using joins; otherwise, you would end up with two CustomerIDs without a direct reference to its corresponding table. Outer Joins Outer joins enable rows to be returned from a join in which one of the tables does not contain matching rows for the other table. Suppose you have two tables that contain the following information: | CustomerID | FirstName | AddressID | | 2 | Zak | 45634 | | 4 | Matthew | 34754 | | 5 | Jessica | | | 10 | David | 97895 | | AddressID | Address | | 45634 | 555 Sample St., San Diego | | 34754 | 343 Chestnut Rd., San Diego | | 97895 | 523 Main St., San Diego | If you were to use an inner join on the preceding tables, such as: Select Customers.FirstName, Customers.AddressID, Address.AddressID, Address.Address From Customers Inner Join Address On Customers.AddressID = Address.AddressID It would return the following results: | FirstName | AddressID | Address | | Zak | 45634 | 555 Sample St., San Diego | | Matthew | 34754 | 343 Chestnut Rd., San Diego | | David | 97895 | 523 Main St., San Diego | Notice that the record that did not contain an AddressID was excluded. If an outer join were performed, such as: Select Customers.FirstName, Customers.AddressID, Address.AddressID, Address.Address From Customers Outer Join Address On Customers.AddressID = Address.AddressID The results would be slightly different: | FirstName | AddressID | Address | | Zak | 45634 | 555 Sample St., San Diego | | Matthew | 34754 | 343 Chestnut Rd., San Diego | | David | 97895 | 523 Main St., San Diego | | Jessica | | | | 
