The relational database term for this relationship is foreign key. CustomerID is the primary key in Customers, but when it appears in another table, such as Orders, it is referred to as a foreign key. You might wonder why we chose to have two separate tables—why not just store Julie’s address in the Orders table? We’ll explore this in more detail in the next section. Schemas The complete set of the table designs for a database is called the database schema. It is akin to a blueprint for the database. A schema should show the tables along with their columns, the data types of the columns and indicate the primary key of each table and any foreign keys. A schema does not include any data, but you might want to show sample data with your schema to explain what it is for. The schema can be shown as it is in the diagrams we are using, in entity relationship diagrams (which are not covered in this book), or in a text form, such as Customers(CustomerID, Name, Address, City) Orders(OrderID, CustomerID, Amount, Date) Underlined terms in the schema are primary keys in the relation in which they are underlined. Dotted underlined terms are foreign keys in the relation in which they appear with a dotted underline. Relationships Foreign keys represent a relationship between data in two tables. For example, the link from Orders to Customers represents a relationship between a row in the Orders table and a row in the Customers table. Three basic kinds of relationships exist in a relational database. They are classified according to the number of things on each side of the relationship. Relationships can be either one-to- one, one-to-many, or many-to-many. A one-to-one relationship means that there is one of each thing in the relationship. For exam- ple, if we had put addresses in a separate table from Customers, there would be a one-to-one relationship between them. You could have a foreign key from Addresses to Customer or the other way around (both are not required). In a one-to-many relationship, one row in one table is linked to many rows in another table. In this example, one Customer might place many Orders. In these relationships, the table that contains the many rows will have a foreign key to the table with the one row. Here, we have put the CustomerID into the Order table to show the relationship. In a many-to-many relationship, many rows in one table are linked to many rows in another table. For example, if we had two tables, Books and Authors, you might find that one book had been Designing Your Web Database C HAPTER 7 7 DESIGNING YOUR WEB DATABASE 175 10 7842 CH07 3/6/01 3:34 PM Page 175 written by two coauthors, each of whom had written other books, on their own or possibly with other authors. This type of relationship usually gets a table all to itself, so you might have Books, Authors, and Books_Authors. This third table would only contain the keys of the other tables as foreign keys in pairs, to show which authors have been involved with which books. How to Design Your Web Database Knowing when you need a new table and what the key should be can be something of an art. You can read huge reams of information about entity relationship diagrams and database nor- malization, which are beyond the scope of this book. Most of the time, however, you can fol- low a few basic design principles. Let’s consider these in the context of Book-O-Rama. Think About the Real World Objects You Are Modeling When you create a database, you are usually modeling real-world items and relationships and storing information about those objects and relationships. Generally, each class of real-world objects you model will need its own table. Think about it: We want to store the same information about all our customers. If there is a set of data that has the same “shape,” we can easily create a table corresponding to that data. In the Book-O-Rama example, we want to store information about our customers, the books that we sell, and details of the orders. The customers all have a name and address. The orders have a date, a total amount, and a set of books that were ordered. The books have an ISBN, an author, a title, and a price. This suggests we need at least three tables in this database: Customers, Orders, and Books. This initial schema is shown in Figure 7.3. At present, we can’t tell from the model which books were ordered in each order. We will deal with this in a minute. Avoid Storing Redundant Data Earlier, we asked the question: “Why not just store Julie Smith’s address in the Orders table?” If Julie orders from Book-O-Rama on a number of occasions, which we hope she will, we will end up storing her data multiple times. You might end up with an Orders table that looks like the one shown in Figure 7.4. There are two basic problems with this. The first is that it’s a waste of space. Why store Julie’s details three times if we only have to store them once? Using MySQL P ART II 176 10 7842 CH07 3/6/01 3:34 PM Page 176 Designing Your Web Database C HAPTER 7 7 DESIGNING YOUR WEB DATABASE 177 CustomerID CUSTOMERS Name Address City 1 Julie Smith 25 Oak Street Airport West 2 Alan Wong 1/47 Haines Avenue Box Hill 3 Michelle Arthur 357 North Road Yarraville ISBN BOOKS Author Title Price 0-672-31687-8 Michael Morgan Java 2 for Professional Developers 34.99 0-672-31745-1 Thomas Down Installing Debian GNU/Linux 24.99 0-672-31509-2 Pruitt, et al. Teach Yourself GIMP in 24 Hours 24.99 OrderID ORDERS CustomerID Amount Date 1 3 27.50 02-Apr-2000 2 1 12.99 15-Apr-2000 3 2 74.00 19-Apr-2000 4 4 6.99 01-May-2000 OrderID ORDERS CustomerIDAmount Date 12 1199.50 25-Apr-2000 13 143.00 29-Apr-2000 14 115.99 30-Apr-2000 15 123.75 01-May-2000 Name Julie Smith Julie Smith Julie Smith Julie Smith Address 28 Oak Street 28 Oak Street 28 Oak Street 28 Oak Street City Airport West Airport West Airport West Airport West FIGURE 7.3 The initial schema consists of Customers, Orders, and Books. FIGURE 7.4 A database design that stores redundant data takes up extra space and can cause anomalies in the data. The second problem is that it can lead to update anomalies, that is, situations where we change the database and end up with inconsistent data. The integrity of the data is violated and we no longer know which data is correct and which incorrect. This generally leads to losing informa- tion. Three kinds of update anomalies need to be avoided: modification, insertion, and deletion anomalies. If Julie moves house while she has pending orders, we will need to update her address in three places instead of one, doing three times as much work. It is easy to overlook this fact and only change her address in one place, leading to inconsistent data in the database (a very bad thing). These problems are called modification anomalies because they occur when we are trying to modify the database. 10 7842 CH07 3/6/01 3:34 PM Page 177 With this design, we need to insert Julie’s details every time we take an order, so each time we must check and make sure that her details are consistent with the existing rows in the table. If we don’t check, we might end up with two rows of conflicting information about Julie. For example, one row might tell us that Julie lives in Airport West, and another might tell us she lives in Airport. This is called an insertion anomaly because it occurs when data is being inserted. The third kind of anomaly is called a deletion anomaly because it occurs (surprise, surprise) when we are deleting rows from the database. For example, imagine that when an order has been shipped, we delete it from the database. When all Julie’s current orders have been ful- filled, they are all deleted from the Orders table. This means that we no longer have a record of Julie’s address. We can’t send her any special offers, and next time she wants to order some- thing from us, we will have to get her details all over again. Generally you want to design your database so that none of these anomalies occur. Use Atomic Column Values This means that in each attribute in each row, we store only one thing. For example, we need to know what books make up each order. There are several ways we could do this. We could add a column to the Orders table which lists all the books that have been ordered, as shown in Figure 7.5. Using MySQL P ART II 178 OrderID ORDERS CustomerID Amount Date 1 3 27.50 02-Apr-2000 2 1 12.99 15-Apr-2000 3 2 74.00 19-Apr-2000 4 3 6.99 01-May-2000 Books Ordered 0-672-31697-8 0-672-31745-1, 0-672-31509-2 0-672-31697-8 0-672-31745-1, 0-672-31509-2, 0-672-31697-8 FIGURE 7.5 With this design, the Books Ordered attribute in each row has multiple values. This isn’t a good idea for a few reasons. What we’re really doing is nesting a whole table inside one column—a table that relates orders to books. When you do it this way, it becomes more difficult to answer questions like “How many copies of Java 2 for Professional Developers have been ordered?” The system can no longer just count the matching fields. Instead, it has to parse each attribute value to see if it contains a match anywhere inside it. Because we’re really creating a table-inside-a-table, we should really just create that new table. This new table is called Order_Items and is shown in Figure 7.6. This table provides a link between the Orders table and the Books table. This type of table is common when there is a many-to-many relationship between two objects—in this case, one order might consist of many books, and each book can be ordered by many people. 10 7842 CH07 3/6/01 3:34 PM Page 178 Choose Sensible Keys Make sure that the keys you choose are unique. In this case, we’ve created a special key for customers (CustomerID) and for orders (OrderID) because these real-world objects might not naturally have an identifier that can be guaranteed to be unique. We don’t need to create a unique identifier for books—this has already been done, in the form of an ISBN. For Order_Item, you can add an extra key if you want, but the combination of the two attributes OrderID and ISBN will be unique as long as more than one copy of the same book in an order is treated as one row. For this reason, the table Order_Items has a Quantity column. Designing Your Web Database C HAPTER 7 7 DESIGNING YOUR WEB DATABASE 179 OrderID ORDER_ITEMS Quantity 11 22 21 31 ISBN 0-672-31697-8 0-672-31745-1 0-672-31509-2 0-672-31697-8 410-672-31745-1 420-672-31509-2 410-672-31697-8 FIGURE 7.6 This design makes it easier to search for particular books that have been ordered. Think About the Questions You Want to Ask the Database Continuing from the last section, think about what questions you want the database to answer. (Think back to those questions we mentioned at the start of the chapter. For example, what are Book-O-Rama’s bestselling books?) Make sure that the database contains all the data required, and that the appropriate links exist between tables to answer the questions you have. Avoid Designs with Many Empty Attributes If we wanted to add book reviews to the database, there are at least two ways we could do this. These two approaches are shown in Figure 7.7. ISBN BOOKS Author ISBN BOOK_REVIEWS Review Title Price 0-672-31687-8 Michael Morgan Java 2 for Professional Developers 34.99 0-672-31745-1 Thomas Down Installing Debian GNU/Linux 24.99 0-672-31509-2 Pruitt, et al. Teach Yourself GIMP in 24 Hours 24.99 Review FIGURE 7.7 To add reviews, we can either add a Review column to the Books table, or add a table specifically for reviews. 10 7842 CH07 3/6/01 3:34 PM Page 179 The first way means adding a Review column to the Books table. This way, there is a field for the Review to be added for each book. If many books are in the database, and the reviewer doesn’t plan to review them all, many rows won’t have a value in this attribute. This is called having a null value. Having many null values in your database is a bad idea. It wastes storage space and causes problems when working out totals and other functions on numerical columns. When a user sees a null in a table, they don’t know if it’s because this attribute is irrelevant, whether there’s a mistake in the database, or whether the data just hasn’t been entered yet. You can generally avoid problems with many nulls by using an alternate design. In this case, we can use the second design proposed in Figure 7.7. Here, only books with a review are listed in the Book_Reviews table, along with their review. Note that this design is based on the idea of having a single in-house reviewer. We could just as easily let customers author reviews. If we wanted to do this, we could add the CustomerID to the Book_Reviews table. Summary of Table Types You will usually find that your database design ends up consisting of two kinds of table: • Simple tables that describe a real-world object. These might also contain keys to other simple objects where there is a one-to-one or one-to-many relationship. For example, one customer might have many orders, but an order is placed by a single customer. Thus, we put a reference to the customer in the order. • Linking tables that describe a many-to-many relationship between two real objects such as the relationship between Orders and Books. These tables are often associated with some kind of real-world transaction. Web Database Architecture Now that we’ve discussed the internal architecture of your database, we’ll look at the external architecture of a Web database system, and discuss the methodology for developing a Web database system. Architecture The basic operation of a Web server is shown in Figure 7.8. This system consists of two objects: a Web browser and a Web server. A communication link is required between them. A Web browser makes a request of the server. The server sends back a response. This architecture suits a server delivering static pages well. The architecture that delivers a database backed Web site is a little more complex. Using MySQL P ART II 180 10 7842 CH07 3/6/01 3:34 PM Page 180 FIGURE 7.8 The client/server relationship between a Web browser and Web server requires communication. The Web database applications we will build in this book follow a general Web database struc- ture that is shown in Figure 7.9. Most of this structure should already be familiar to you. Designing Your Web Database C HAPTER 7 7 DESIGNING YOUR WEB DATABASE 181 Browser Response Request Web Server Browser 6 1 Web Server 5 2 PHP Engine 4 3 MySQL Server FIGURE 7.9 The basic Web database architecture consists of the Web browser, Web server, scripting engine, and database server. A typical Web database transaction consists of the following stages, which are numbered in Figure 7.9. We will examine the stages in the context of the Book-O-Rama example. 1. A user’s Web browser issues an HTTP request for a particular Web page. For example, she might have requested a search for all the books at Book-O-Rama written by Laura Thomson, using an HTML form. The search results page is called results.php. 2. The Web server receives the request for results.php, retrieves the file, and passes it to the PHP engine for processing. 3. The PHP engine begins parsing the script. Inside the script is a command to connect to the database and execute a query (perform the search for books). PHP opens a connec- tion to the MySQL server and sends on the appropriate query. 4. The MySQL server receives the database query and processes it, and sends the results— a list of books—back to the PHP engine. 5. The PHP engine finishes running the script, which will usually involve formatting the query results nicely in HTML. It then returns the resulting HTML to the Web server. 6. The Web server passes the HTML back to the browser, where the user can see the list of books she requested. The process is basically the same regardless of which scripting engine or database server you use. Often the Web server software, the PHP engine, and the database server all run on the same machine. However, it is also quite common for the database server to run on a different machine. You might do this for reasons of security, increased capacity, or load spreading. From a development perspective, this will be much the same to work with, but it might offer some significant advantages in performance. 10 7842 CH07 3/6/01 3:34 PM Page 181 Further Reading In this chapter, we covered some guidelines for relational database design. If you want to delve into the theory behind relational databases, you can try reading books by some of the relational gurus like C.J. Date. Be warned, however, that the material can get pretty theoretical and might not be immediately relevant to a commercial Web developer. Your average Web database tends not to be that complicated. Next In the next chapter, we’ll start setting up your MySQL database. First you’ll learn how to set up a MySQL database for the Web, how to query it, and then how to query it from PHP. Using MySQL P ART II 182 10 7842 CH07 3/6/01 3:34 PM Page 182 CHAPTER 8 Creating Your Web Database 11 7842 CH08 3/6/01 3:38 PM Page 183 Using MySQL P ART II 184 In this chapter we’ll talk about how to set up a MySQL database for use on a Web site. We’ll cover • Creating a database • Users and privileges • Introduction to the privilege system • Creating database tables • Column types in MySQL In this chapter, we’ll follow through with the Book-O-Rama online bookstore application dis- cussed in the last chapter. As a reminder, here is the schema for the Book-O-Rama application: Customers(CustomerID, Name, Address, City) Orders(OrderID, CustomerID, Amount, Date) Books(ISBN, Author, Title, Price) Order_Items(OrderID, ISBN, Quantity) Book_Reviews(ISBN, Reviews) Remember that primary keys are underlined and foreign keys have a dotted underline. In order to use the material in this section, you must have access to MySQL. This usually means that you 1. Have completed the basic install of MySQL on your Web server. This includes • Installing the files • Setting up a user for MySQL to run as • Setting up your path • Running mysql_install_db, if required • Setting the root password • Deleting the anonymous user • Starting the MySQL server and setting it up to run automatically If you’ve done all those things, you can go right ahead and read this chapter. If you haven’t, you can find instructions on how to do these things in Appendix A, “Installing PHP 4 and MySQL.” If you have problems at any point in this chapter, it might be because your MySQL sys- tem is not set up correctly. If that happens, refer back to this list and Appendix A to make sure that your set up is correct. 11 7842 CH08 3/6/01 3:38 PM Page 184 . DATABASE 179 OrderID ORDER_ITEMS Quantity 11 22 21 31 ISBN 0-6 7 2-3 169 7-8 0-6 7 2-3 174 5-1 0-6 7 2-3 150 9-2 0-6 7 2-3 169 7-8 41 0-6 7 2-3 174 5-1 42 0-6 7 2-3 150 9-2 41 0-6 7 2-3 169 7-8 FIGURE 7.6 This design makes it easier to search for particular books. 7.5. Using MySQL P ART II 178 OrderID ORDERS CustomerID Amount Date 1 3 27.50 02-Apr-2000 2 1 12.99 15-Apr-2000 3 2 74.00 19-Apr-2000 4 3 6.99 01-May-2000 Books Ordered 0-6 7 2-3 169 7-8 0-6 7 2-3 174 5-1 , 0-6 7 2-3 150 9-2 0-6 7 2-3 169 7-8 0-6 7 2-3 174 5-1 ,. 3 27.50 02-Apr-2000 2 1 12.99 15-Apr-2000 3 2 74.00 19-Apr-2000 4 4 6.99 01-May-2000 OrderID ORDERS CustomerIDAmount Date 12 1199.50 25-Apr-2000 13 143.00 29-Apr-2000 14 115.99 30-Apr-2000 15