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The Language of SQL- P26

The Language of SQL- P26:Research has shown that, being pressed for time, most readers tend to skip the introduction of any book they happen to read and then proceed immediately to the first real chapter. With that fact firmly in mind, we will only cover relatively unimportant material in the introduction, such as an explanation of what you will and will not learn by reading this book. | Joining Two Tables 111 The Orders table would be similar to this OrderlD CustomerlD Quantity PricePerItem 1 1 4 2.50 2 2 10 1.25 3 2 12 1.50 4 3 5 4.00 We ll be using these two tables for the examples in this chapter. Notice a number of additions. The Customers table now contains information only about customers. The Orders table now has information only about items purchased. We added a CustomerID column to the Orders table to tell which customer placed the order. As you might remember from Chapter 1 this is referred to as a foreign key. We also added a row to the Orders table to indicate one customer Natalie Lopez who placed more than one order. Additionally we also added a new row to the Customers table to represent a potential customer Adam Petrie who has not yet placed an order. Of course there s much information that is still missing. For example an Orders table would typically include additional columns such as one that stores the order date. Also an Orders table would typically have a foreign key column with a product ID so the order could be linked to information about the product that was sold. Plus the Orders table itself might in fact be split into more than one table so that information about the entire order such as order date could be stored separately from information about each item that was ordered assuming that a customer could order more than one item in an order . In other words this still is not a completely realistic example. But now that we ve split our information into two separate tables we can address how to create a SELECT statement that can pull data from both tables simultaneously. Before we get to the SELECT statement itself we need to address one additional concern which is how to represent visually the two tables and the implied relationship that exists between them. Previously we displayed each table with the column names on the top row and corresponding data on subsequent rows. Now that we have more than one table to deal with we