An Introduction to Databases
As I've already explained, PHP is a server-side scripting language that lets you insert into your Web pages instructions that your Web server software (be it Apache, IIS, or whatever) will execute before it sends those pages to browsers that request them. In a brief example, I showed how it was possible to insert the current date into a Web page every time it was requested.

Now, that's all well and good, but things really get interesting when a database is added to the mix. A database server (in our case, MySQL) is a program that can store large amounts of information in an organized format that's easily accessible through scripting languages like PHP. For example, you could tell PHP to look in the database for a list of jokes that you'd like to appear on your Website.

In this example, the jokes would be stored entirely in the database. The advantages of this approach would be twofold. First, instead of having to write an HTML file for each of your jokes, you could write a single PHP file that was designed to fetch any joke from the database and display it. Second, adding a joke to your Website would be a simple matter of inserting the joke into the database. The PHP code would take care of the rest, automatically displaying the new joke along with the others when it fetched the list from the database.

Let's run with this example as we look at how data is stored in a database. A database is composed of one or more tables, each of which contains a list of things. For our joke database, we'd probably start with a table called joke that would contain a list of jokes. Each table in a database has one or more columns, or fields. Each column holds a certain piece of information about each item in the table. In our example, our joke table might have one column for the text of the jokes, and another for the dates on which the jokes were added to the database. Each joke stored in this way would then be said to be a row in the table. These rows and columns form a table that looks like Figure 2.1.

Notice that, in addition to columns for the joke text (joketext) and the date of the joke (jokedate), I included a column named id. As a matter of good design, a database table should always provide a means by which we can identify each of its rows uniquely. Since it's possible that a single joke could be entered more than once on the same date, the joketext and jokedate columns can't be relied upon to tell all the jokes apart. The function of the id column, therefore, is to assign a unique number to each joke so that we have an easy way to refer to them and to keep track of which joke is which. Such database design issues will be covered in greater depth in Chapter 5, Relational Database Design.

So, to review, the above is a three-column table with two rows, or entries. Each row in the table contains three fields, one for each column in the table: the joke's ID, its text, and the date of the joke. With this basic terminology under our belts, we're ready to get started with MySQL.

Logging On to MySQL
The standard interface for working with MySQL databases is to connect to the MySQL server software (which you set up in Chapter 1, Installation) and type commands one at a time. To make this connection to the server, you'll need the MySQL client program. If you installed the MySQL server software yourself, either under Windows or some brand of UNIX, this program will have been installed in the same location as the server program. Under Linux, for example, the program is called mysql and is located by default in the /usr/local/mysql/bin directory. Under Windows, the program is called mysql.exe and is located by default in the C:\mysql\bin directory.

If you didn't set up the MySQL server yourself (if, for example, you're working on your Web host's MySQL server), there are two ways to connect to the MySQL server. The first is to use Telnet or a Secure Shell (SSH) connection to log into your Web host's server, then run mysql from there. The second is to download the MySQL client software from http://www.mysql.com/ (available free for Windows and Linux), install it on your own computer, and use it to connect to the MySQL server over the Internet. Both methods work well, and your Web host may support one, the other, or both—you'll need to ask.

No shell? No direct connection? No problem!

Many Web hosts do not allow direct access to their MySQL servers over the Internet for security reasons. If your host has adopted this policy (you'll have to ask them if you're not sure), installing the MySQL client software on your own computer won't do you any good. Instead, you'll need to install a Web-based MySQL administration script onto your site. phpMyAdmin is the most popular script available; indeed, many Web hosts will configure your account with a copy of phpMyAdmin.

While Web-based MySQL administration systems provide a convenient, graphical interface for working with your MySQL databases, it is still important to learn the basics of MySQL's command-line interface. The commands you use in this interface are the very same commands you'll have to include in your PHP code later in this book. I therefore recommend going back to Chapter 1, Installation and installing MySQL on your own computer so you can complete the exercises in this chapter before you get comfortable with your Web-based administration interface.

Whichever method and operating system you use, you'll end up at a command prompt, ready to run the MySQL client program and connect to your MySQL server. Here's what you should type:

mysql -h hostname –u username -p

You need to replace hostname with the host name or IP address of the computer on which the MySQL server is running. If the client program is run on the same computer as the server, you would use -h localhost or –h 127.0.0.1, but in this special case you can actually leave off this part of the command entirely. username should be your MySQL user name. If you installed the MySQL server yourself, this will just be root. If you're using your Web host's MySQL server, this should be the MySQL user name the host assigned you.

The -p argument tells the program to prompt you for your password, which it should do as soon as you enter the command above. If you set up the MySQL server yourself, this password is the root password you chose in Chapter 1, Installation. If you're using your Web host's MySQL server, this should be the MySQL password the host gave you.

If you typed everything correctly, the MySQL client program will introduce itself and dump you on the MySQL command prompt:

mysql>

The MySQL server can actually keep track of more than one database. This allows a Web host to set up a single MySQL server for use by several of its subscribers, for example. So, your next step should be to choose a database with which to work. First, let's retrieve a list of databases on the current server. Type this command (don't forget the semicolon!) and press Enter.

mysql>SHOW DATABASES;

MySQL will show you a list of the databases on the server. If you're working on a brand new server (i.e. if you installed the server yourself in Chapter 1), the list should look like this:

+----------+
| Database |
+----------+
| mysql    |
| test     |
+----------+
2 rows in set (0.11 sec)

The MySQL server uses the first database, named mysql, to keep track of users, their passwords, and what they're allowed to do. We'll steer clear of this database for now, though we will revisit it in Chapter 8, MySQL Administration, when we discuss MySQL Administration. The second database, named test, is a sample database. You can actually get rid of this database. I won't be referring to it in this book, and we'll create our own example database momentarily. Deleting something in MySQL is called "dropping" it, and the command for doing so is appropriately named:

mysql>DROP DATABASE test;

If you type this command and press Enter, MySQL will obediently delete the database, displaying "Query OK" in confirmation. Notice that you're not prompted with any kind of "Are you sure?" message. You have to be very careful to type your commands correctly in MySQL because, as this example shows, you can obliterate your entire database—along with all the information it contains—with a single command!

Before we go any further, let's learn a couple of things about the MySQL command prompt. As you may have noticed, all commands in MySQL are terminated by a semicolon (. If you forget the semicolon, MySQL will think you haven't finished typing your command, and will let you continue to type on another line:

mysql>SHOW
   ->DATABASES;

MySQL shows that it's waiting for you to type more of your command by changing the prompt from mysql> to ->. This handy functionality allows you to spread long commands over several lines.

If you get halfway through a command and realize that you made a mistake early on, you may want to cancel the current command entirely and start over from scratch. To do this, type \c and press Enter:

mysql>DROP DATABASE\c
mysql>

MySQL will ignore completely the command you had begun to type and will return to the prompt to await another command.

Finally, if at any time you want to exit the MySQL client program, just type quit or exit (either will work). This is the only command that doesn't need a semicolon, but you can use one if you want to.

mysql>quit
Bye

So, What's SQL?
The set of commands we'll use to direct MySQL throughout the rest of this book is part of a standard called Structured Query Language, or SQL (pronounced either "sequel" or "ess-cue-ell"—take your pick). Commands in SQL are also referred to as queries (I'll use these two terms interchangeably).

SQL is the standard language for interacting with most databases, so, even if you move from MySQL to a database like Microsoft SQL Server in the future, you'll find that most of the commands are identical. It's important that you understand the distinction between SQL and MySQL. MySQL is the database server software that you're using. SQL is the language that you use to interact with that database.

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Creating a Database
Those who are working on their Web host's MySQL server are likely already to have been assigned a database with which to work. Sit tight; we'll get back to you in a moment. If you're running a MySQL server that you installed yourself, however, you'll need to create your own database. It's just as easy to create a database as it is to delete one:

mysql>CREATE DATABASE ijdb;

I chose to name the database ijdb, for Internet Joke Database, because that fits with the example we're using. Feel free to give the database any name you like, though. Those of you working on your Web host's MySQL server will probably have no choice in what to name your database, as it will probably already have been created for you.

Now that we have a database, we need to tell MySQL that we want to use it. Again, the command isn't difficult to remember:

mysql>USE ijdb;

You're now ready to use your database. Since a database is empty until you add some tables to it, our first order of business will be to create a table that will hold our jokes.

Creating a Table
The SQL commands we've encountered so far have been reasonably simple, but as tables are so flexible, it takes a more complicated command to create them. The basic form of the command is as follows:

mysql>CREATE TABLE table_name (
   ->  column_1_name column_1_type column_1_details,
   ->  column_2_name column_2_type column_2_details,
   ->  ...
   ->);

Let's return to our example joke table. Recall that it had three columns: id (a number), joketext (the text of the joke), and jokedate (the date on which the joke was entered). The command to create this table is as follows:

mysql>CREATE TABLE joke (
   ->  id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
   ->  joketext TEXT,
   ->  jokedate DATE NOT NULL
   ->);

It looks pretty scary, huh? Let's break it down:


The first line is fairly simple; it says that we want to create a new table named joke.
The second line says that we want a column called id that will contain an integer (INT), that is, a whole number. The rest of this line deals with special details for the column. First, this column is not allowed to be left blank (NOT NULL). Next, if we don't specify any value in particular when we add a new entry to the table, we want MySQL to pick a value that is one more than the highest value in the table so far (AUTO_INCREMENT). Finally, this column is to act as a unique identifier for the entries in the table, so all values in this column must be unique (PRIMARY KEY).
The third line is super-simple; it says that we want a column called joketext, which will contain text (TEXT).
The fourth line defines our last column, called jokedate, which will contain data of type DATE, and which cannot be left blank (NOT NULL).

Note that, while you're free to type your SQL commands in upper– or lowercase, a MySQL server running on a UNIX-based system will be case-sensitive when it comes to database and table names, as these correspond to directories and files in the MySQL data directory. Otherwise, MySQL is completely case-insensitive, but for one exception: table, column, and other names must be spelled exactly the same when they're used more than once in the same command.

Note also that we assigned a specific type of data to each column we created. id will contain integers, joketext will contain text, and jokedate will contain dates. MySQL requires you to specify in advance a data type for each column. Not only does this help keep your data organized, but it allows you to compare the values within a column in powerful ways, as we'll see later. For a complete list of supported MySQL data types, see Appendix C, MySQL Column Types.

Now, if you typed the above command correctly, MySQL will respond with Query OK, and your first table will be created. If you made a typing mistake, MySQL will tell you there was a problem with the query you typed, and will try to indicate where it had trouble understanding what you meant.

For such a complicated command, Query OK is a pretty boring response. Let's have a look at your new table to make sure it was created properly. Type the following command:

mysql>SHOW TABLES;

The response should look like this:

+----------------+
| Tables in ijdb |
+----------------+
| joke           |
+----------------+
1 row in set

This is a list of all the tables in our database (which I named ijdb above). The list contains only one table: the joke table we just created. So far, everything seems fine. Let's take a closer look at the joke table itself:

mysql>DESCRIBE joke;
+----------+---------+------+-----+------------+----------------+
| Field    | Type    | Null | Key | Default    | Extra          |
+----------+---------+------+-----+------------+----------------+
| id       | int(11) |      | PRI | NULL       | auto_increment |
| joketext | text    | YES  |     | NULL       |                |
| jokedate | date    |      |     | 0000-00-00 |                |
+----------+---------+------+-----+------------+----------------+
3 rows in set

As you can see, there are three columns (or fields) in this table, which appear as the three rows in this table of results. The details are somewhat cryptic, but if you look at them closely, you should be able to figure out what they mean. Don't worry about it too much, though. We've got better things to do, like adding some jokes to our table!

We need to look at just one more thing before we get to that, though: deleting a table. This task is as frighteningly easy as deleting a database. In fact, the command is almost identical:

mysql>DROP TABLE tableName;

Inserting Data into a Table
Our database is created and our table is built; all that's left is to put some actual jokes into the database. The command that inserts data into a database is called, appropriately enough, INSERT. This command takes two basic forms:

mysql>INSERT INTO table_name SET
   -> columnName1 = value1,
   -> columnName2 = value2,
   -> ...
   ->;

mysql>INSERT INTO table_name
   -> (columnName1, columnName2, ...)
   -> VALUES (value1, value2, ...);

So, to add a joke to our table, we can use either of these commands:

mysql>INSERT INTO joke SET
   ->joketext = "Why did the chicken cross the road? To get to
   "> the other side!",
   ->jokedate = "2004-04-01";

mysql>INSERT INTO joke
   ->(joketext, jokedate) VALUES (
   ->"Why did the chicken cross the road? To get to the other
   "> side!",
   ->"2004-04-01"
   ->);

Note that in the second form of the INSERT command, the order in which you list the columns must match the order in which you list the values. Otherwise, the order of the columns doesn't matter, as long as you provide values for all required fields. Now that you know how to add entries to a table, let's see how we can view those entries.

Viewing Stored Data
The command we use to view data stored in database tables, SELECT, is the most complicated command in the SQL language. The reason for this complexity is that the chief strength of a database is its flexibility in data retrieval and presentation. At this early point in our experience with databases we need only fairly simple lists of results, so we'll just consider the simpler forms of the SELECT command here. This command will list everything that's stored in the joke table:

mysql>SELECT * FROM joke;

Read aloud, this command says "select everything from joke." If you try this command, your results will resemble the following:

+----+---------------------------------------------------
------------+------------+
| id | joketext                                          
           | jokedate   |
+----+---------------------------------------------------
------------+------------+
|  1 | Why did the chicken cross the road? To get to the  
other side! | 2004-04-01 |
+----+---------------------------------------------------
------------+------------+
1 row in set (0.05 sec)

The results look a little disorganized because the text in the joketext column is so long that the table can't fit on the screen properly. For this reason, you might want to tell MySQL to leave out the joketext column. The command for doing this is as follows:

mysql>SELECT id, jokedate FROM joke;

This time, instead of telling it to "select everything," we told it precisely which columns we wanted to see. The results look like this:

+----+------------+
| id | jokedate   |
+----+------------+
| 1  | 2004-04-01 |
+----+------------+
1 row in set (0.00 sec)

Not bad, but we'd like to see at least some of the joke text, wouldn't we?

Source: http://www.sitepoint.com/artic...ed-mysql/2

As well as being able to name specific columns that we want the SELECT command to show us, we can use functions to modify each column's display. One function, called LEFT, lets us tell MySQL to display a column's contents up to a specified maximum number of characters. For example, let's say we wanted to see only the first 20 characters of the joketext column. Here's the command we'd use:

mysql>SELECT ID, LEFT(joketext, 20), jokedate FROM joke;
+----+----------------------+------------+
| id | LEFT(joketext, 20)   | jokedate   |
+----+----------------------+------------+
| 1  | Why did the chicken  | 2004-04-01 |
+----+----------------------+------------+
1 row in set (0.05 sec)

See how that worked? Another useful function is COUNT, which lets us count the number of results returned. If, for example, we wanted to find out how many jokes were stored in our table, we could use the following command:

mysql>SELECT COUNT(*) FROM joke;
+----------+
| COUNT(*) |
+----------+
| 1        |
+----------+
1 row in set (0.06 sec)

As you can see, we have just one joke in our table and, so far, all the examples have fetched all the entries in our table. However, we can limit our results to include only those database entries that have the specific attributes we want. We set these restrictions by adding what's called a WHERE clause to the SELECT command. Consider this example:

mysql>SELECT COUNT(*) FROM joke WHERE jokedate >= "2004-01-01";

This query will count the number of jokes that have dates greater than or equal to January 1, 2004. In the case of dates, "greater than or equal to" means "on or after." Another variation on this theme lets you search for entries that contain a certain piece of text. Check out this query:

mysql>SELECT joketext FROM joke WHERE joketext LIKE "%chicken%";

The above query displays the text of all jokes that contain the word "chicken" in their joketext column. The LIKE keyword tells MySQL that the named column must match the given pattern. In this case, the pattern we've used is "%chicken%". The % signs indicate that the word "chicken" may be preceded and/or followed by any string of text.

Additional conditions may also be combined in the WHERE clause to further restrict results. For example, to display knock-knock jokes from April 2004 only, we could use the following query:

mysql>SELECT joketext FROM joke WHERE
   ->joketext LIKE "%knock%" AND
   ->jokedate >= "2004-04-01" AND
   ->jokedate < "2004-05-01";

Enter a few more jokes into the table and experiment with SELECT statements. A good familiarity with the SELECT statement will come in handy later in this book.

You can do a lot with the SELECT statement. We'll look at some of its more advanced features later, when we need them.

Modifying Stored Data
Having entered your data into a database table, you might like to change it. Whether you want to correct a spelling mistake, or change the date attached to a joke, such alterations are made using the UPDATE command. This command contains elements of the INSERT command that set column values, and elements of the SELECT command that pick out entries for modification. The general form of the UPDATE command is as follows:

mysql>UPDATE table_name SET
   ->  col_name = new_value, ...
   ->WHERE conditions;

So, for example, if we wanted to change the date on the joke we entered above, we'd use the following command:

mysql>UPDATE joke SET jokedate="1994-04-01" WHERE id=1;

Here's where that id column comes in handy: it allows us to easily single out a joke for changes. The WHERE clause used here works just as it did in the SELECT command. This next command, for example, changes the date of all entries that contain the word "chicken:"

mysql>UPDATE joke SET jokedate="1994-04-01"
   ->WHERE joketext LIKE "%chicken%";

Deleting Stored Data
The deletion of entries in SQL is dangerously easy, which, if you haven't noticed yet, is a recurring theme. Here's the command syntax:

mysql>DELETE FROM table_name WHERE conditions;

To delete all chicken jokes from your table, you'd use the following query:

mysql>DELETE FROM joke WHERE joketext LIKE "%chicken%";

One thing to note is that the WHERE clause is actually optional. You should be very careful, however, if you leave it out, as the DELETE command will then apply to all entries in the table. This command will empty the joke table in one fell swoop:

mysql>DELETE FROM joke;

Scary, huh?

Summary
There's a lot more to the MySQL database system and the SQL language than the few basic commands we've discussed here, but these commands are by far the most commonly used. To date, we've only worked with a single table, but to realize the true power of a relational database, we'll also need to learn how to use multiple tables together to represent potentially complex relationships between database entities.

We'll cover all this and more in Chapter 5, Relational Database Design, where we'll discuss database design principles and look at some more advanced examples. For now, though, we've accomplished our objective, and you can comfortably interact with MySQL using the command line interface. In Chapter 3, Getting Started with PHP, the fun continues as we delve into the PHP server-side scripting language, and use it to create dynamic Web pages. If you like, you can practice with MySQL a little before you move on by creating a decent-sized joke table. This knowledge will come in handy in /#l#/http://www.sitepoint.com/article/publishing-mysql-data-web/#lt#/Chapter 4, Publishing MySQL Data on the Web.

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