Getting started with MySQL
by Russell J.T. Dyer
Over the past few years, MySQL has become one of the most popular database systems. The company which develops the software, MySQL AB of Sweden, is experiencing about 35,000 downloads a day and estimates over 5 million installations worldwide.
Since MySQL can be selected from installation as part of Red Hat? Enterprise Linux? and Fedora Core, I won't explain how to install the software. However, if you don't have MySQL installed on your server, you can either get it from your Linux installation CDs or you can go to MySQL's download page to get the latest RPMs. Just be sure to use the latest, recommended Generally Available release and install the Standard distribution of each RPM for your type of system.
Connecting to MySQL
MySQL is a database system, a database server. To interface with the MySQL server, you can use a client program or you can write a program or script with one of the popular programming languages (such as Perl) using an API (Application Programming Interface) to interface with the MySQL server. This article focuses on using the default client that comes with MySQL called mysql. With this client, you can either enter queries from the command line or you can switch to a terminal, that is to say, monitor mode. We'll use the latter.
From the command line, enter the following to log in as the root user and to enter monitor mode:
mysql -u root -p -h localhost
The -u option is for specifying the user name. Replace root if you want to use a different user name. This is the MySQL user name, not the Linux user name. The password for the MySQL user root is probably different from the Linux user root. Incidentally, it's not a good security practice to use the root user unless you have a specific administrative task to perform for which only root has the needed privileges. The -p option above instructs the mysql client to prompt for the password. If the password for the root user hasn't been set yet, then the password is blank and you would just hit Enter when prompted. The -h option is for specifying the host name or the IP address of the server. This would be necessary if the client is running on a different machine than the server. If you've secure-shelled into the server machine, you probably won't need to use the host option. In fact, if you're logged into Linux as root, you won't need the user option—the -p is all you'll need. Once you've entered the line above along with the password when prompted, you are logged into MySQL through the client. To exit, type quit or exit and press Enter.
Creating a structure
To be able to add and manipulate data, first you have to create a database structure. Creating a database is simple. Enter something like the following from within the mysql client:
CREATE DATABASE bookstore; USE bookstore;
This very minimal, first SQL statement creates a sub-directory called bookstore on the Linux file system in the directory which holds your MySQL data files. It won't create any data, obviously. It just sets up a place to add tables, which in turn hold data. The second SQL statement above sets this new database as the default database. It will remain your default until you change it or until you log out of MySQL.
The next step is to begin creating tables. This is only a little more complicated. To create a simple table that holds basic data on books, enter the following:
CREATE TABLE books ( isbn CHAR(20) PRIMARY KEY, title VARCHAR(50), author_id INT, publisher_id INT, year_pub CHAR(4), description TEXT );
This SQL statement creates the table books with six fields, or rather columns. The first column (isbn) is an identification number for each row—this name relates to the unique identifier used in the book publishing business. It has a fixed width character type of 20 characters. It will be the primary key column on which data will be indexed. The column data type for the book title is a variable width character column of fifty characters at most. The third and fourth columns will be used for identification numbers for the author and the publisher. They are integer data types. The fifth column is used for the publication year of each book. The last column is for entering a description of each book. It's a TEXT data type, which means that it's a variable width column and it can hold up 65535 bytes of data for each row. There are several other data types that may be used for columns, but this gives you a good sampling.
To see how the table we created looks, enter the following SQL statement:
DESCRIBE books;
which produces the following output:
+--------------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------------+-------------+------+-----+---------+-------+ | isbn | varchar(20) | | PRI | | | | title | varchar(50) | YES | | NULL | | | author_id | int(11) | YES | | NULL | | | publisher_id | int(11) | YES | | NULL | | | year_pub | char(4) | YES | | NULL | | | description | text | YES | | NULL | | +--------------+-------------+------+-----+---------+-------+
To change the settings of a table, you can use the ALTER TABLE statement. I'll cover that statement in another article. To delete a table completely (including its data), you can use the DROP TABLE statement, followed by the table name. Be careful with this statement since it's not reversible.
The next table we'll create for our examples is the authors table to hold author information. This table saves us from having to enter the author's name and other related data for each book written by each author. It also helps to ensure consistency of data: there's less chance of inadvertent spelling deviations.
CREATE TABLE authors ( author_id INT AUTO_INCREMENT PRIMARY KEY, name_last VARCHAR(50), name_first VARCHAR(50), country VARCHAR(50) );
We'll join this table to the books table as needed. For instance, we would use it when we want a list of books along with their corresponding authors' names. For a real bookstore's database, both of these tables would probably have more columns. There would also be several more tables. For the examples that follow, these two tables are enough.
Minor items
Before moving on to the next step of adding data to the tables, let me point out a few minor items that I've omitted mentioning. SQL statements end with a semi-colon (or a \G). You can spread an SQL statement over multiple lines. However, it won't be passed to the server by the client until you terminate it with a semi-colon and hit Enter. To cancel an SQL statement once you've started typing it, enter \c and press Enter.
As a basic convention, reserved words are printed in all capital letters. This isn't necessary, though. MySQL is case-insensitive with regards to reserved words. Database and table names, however, are case-sensitive on Linux. This is because they reference the related directories and files on the file system. Column names aren't case sensitive since they're not affected by the file system, per se. As another convention, I use lower-case letters for structural names (such as table names). It's a matter of preference for deciding on names.
Entering data
The primary method for entering data into a table is to use the INSERT statement. As an example, let's enter some information about an author into the authors table:
INSERT INTO authors
(name_last, name_first, country)
VALUES('Kafka', 'Franz', 'Czech Republic');
This adds the name and country of the author Franz Kafka to the authors table. We don't need to give a value for the author_id since that column was created with the AUTO_INCREMENT flag. MySQL automatically assigns an identification number. You can manually assign one, especially if you want to start the count at a higher number than 1 (such as 1000). Since we are not providing data for all of the columns in the table, we have to list the columns for which we are giving data and in the order that the data is given in the set following the VALUES keyword. This means that we could give the data in a different order.
For an actual database, we would probably enter data for many authors. We'll assume that we've done that and move on to entering data for some books. Below is an entry for one of Kafka's books:
INSERT INTO books
(title, author_id, isbn, year_pub)
VALUES('The Castle', '1', '0805211063', '1998');
This adds a record for Kafka's book, The Castle. Notice that I mixed up the order of the columns, but it still works because both sets agree. We indicate that the author is Kafka by giving a value of 1 for the author_id. This is the value that was assigned by MySQL when we entered the row for Kafka earlier. Let's enter a few more books for Kafka, but by a different method:
INSERT INTO books
(title, author_id, isbn, year_pub)
VALUES('The Trial', '1', '0805210407', '1995'),
('The Metamorphosis', '1', '0553213695', '1995'),
('America', '1', '0805210644', '1995');
In this example, we've added three books in one statement. This allows us to give the list of column names once. We also give the keyword VALUES only once, followed by a separate set of values for each book, each contained in parentheses and separated by commas. This cuts down on typing and speeds up the process. Either method is fine, and both have their advantages. To be able to continue with our examples, let's assume that data on thousands of books has been entered. With that behind us, let's look at how to retrieve data from tables.
Retrieving data
The primary method of retrieving data from tables is to use a SELECT statement. There are many options available with the SELECT statement, but you can start simply. As an example, let's retrieve a list of book titles from the books table:
SELECT title
FROM books;
This displays all of the rows of books in the table. If the table has thousands of rows, MySQL displays thousands. To limit the number of rows retrieved, add a LIMIT clause to the SELECT statement:
SELECT title
FROM books
LIMIT 5;
This limits the number of rows displayed to five. To be able to list the author's name for each book along with the title, you have to join the books table with the authors table. To do this, use the JOIN clause:
SELECT title, name_last
FROM books
JOIN authors USING (author_id);
Notice that the primary table from which we're drawing data is given in the FROM clause. The table to which we're joining is given in the JOIN clause along with the commonly named column (author_id) that we're using for the join. For older versions of MySQL, you may have to join the tables differently.
To retrieve the titles of only books written by Kafka based on his name (not the author_id), we would use the WHERE clause with the SELECT statement:
SELECT title AS 'Kafka Books'
FROM books
JOIN authors USING (author_id)
WHERE name_last = 'Kafka';
+-------------------+
| Kafka Books |
+-------------------+
| The Castle |
| The Trial |
| The Metamorphosis |
| America |
+-------------------+
This statement lists the titles of Kafka books stored in the database. Notice that I've added the AS parameter next to the column name title to change the column heading in the results set to Kafka Books. This is known as an alias. Looking at the results here, we can see that the title for one of Kafka's books is incorrect. His book Amerika is spelled above with a "c" in the table instead of a "k". This leads to the next section on changing data.
Changing and deleting data
To change existing data, a common method is to use the UPDATE statement. When changing data, though, we need to be sure that we change the correct rows. In our example, there could be another book with the title America written by a different author. Since the key column isbn has only unique numbers and we know the ISBN number for the book that we want to change, we can use it to specify the row.
UPDATE books
SET title = 'Amerika'
WHERE isbn = '0805210644';
This changes the value of the title column for the row specified. We could change the value of other columns for the same row by giving the column = value for each, separated by commas.
If we want to delete a row of data, we can use the DELETE statement. For instance, suppose that our fictitious bookstore has decided no longer to carry books by John Grisham. By first running a SELECT statement, we determine the identification number for the author to be 2034. Using this author identification number, we could enter the following:
DELETE FROM books
WHERE author_id = '2034';
This statement deletes all rows from the table books for the author_id given. To do a clean job of it, we'll have to do the same for the authors table. We would just replace the table name in the statement above; everything else would be the same.
Conclusion
This is a very basic primer for using MySQL. Hopefully, it gives you the idea of how to get started with MySQL. Each of the SQL statements mentioned here have several more options and clauses each. You can learn more about them from experimenting and by reading the on-line documentation provided by MySQL AB.