Connecting to MariaDB
MariaDB is a database system, a database server. To interface with the MariaDB server, you can use a client program, or you can write a program or script with one of the popular programming languages (e.g., PHP) using an API (Application Programming Interface) to interface with the MariaDB server. For the purposes of this article, we will focus on using the default client that comes with MariaDB called
mariadb. 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. To start, we'll use the latter.
From the Linux command-line, you would enter the following to log in as the root user and to enter monitor mode:
mariadb -u root -p -h localhost
-u option is for specifying the user name. You would replace
root here if you want to use a different user name. This is the MariaDB user name, not the Linux user name. The password for the MariaDB user
root will probably be 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.
-p option above instructs the
mariadb client to prompt you 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 will be logged into MariaDB through the client. To exit, type quit or exit and press [Enter].
Creating a Structure
In order to be able to add and to manipulate data, you first have to create a database structure. Creating a database is simple. You would enter something like the following from within the mariadb client:
CREATE DATABASE bookstore; USE bookstore;
This very minimal, first SQL statement will create a sub-directory called bookstore on the Linux filesystem in the directory which holds your MariaDB data files. It won't create any data, obviously. It'll just set up a place to add tables, which will in turn hold data. The second SQL statement above will set this new database as the default database. It will remain your default until you change it to a different one or until you log out of MariaDB.
The next step is to begin creating tables. This is only a little more complicated. To create a simple table that will hold basic data on books, we could enter something like 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 to 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; +--------------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------------+-------------+------+-----+---------+-------+ | isbn | char(20) | NO | PRI | NULL | | | 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 will save 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 as they are will be enough.
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. MariaDB 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 filesystem. Column names aren't case sensitive since they're not affected by the filesystem, per se. As another convention, we use lower-case letters for structural names (e.g., table names). It's a matter of preference for deciding on names.
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. We'll do that like so:
INSERT INTO authors (name_last, name_first, country) VALUES('Kafka', 'Franz', 'Czech Republic');
This will add 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 option. MariaDB will automatically assign an identification number. You can manually assign one, especially if you want to start the count at a higher number than 1 (e.g., 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 we 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 MariaDB 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.
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 will display all of the rows of books in the table. If the table has thousands of rows, MariaDB will display thousands. To limit the number of rows retrieved, we could add a LIMIT clause to the SELECT statement like so:
SELECT title FROM books LIMIT 5;
This will limit the number of rows displayed to five. To be able to list the author's name for each book along with the title, you will have to join the books table with the authors table. To do this, we can use the JOIN clause like so:
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 (i.e., author_id) that we're using for the join.
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. This would be entered like the following:
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 will list 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 & Deleting Data
In order 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 will change 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 will delete 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.
This is a very basic primer for using MariaDB. Hopefully, it gives you the idea of how to get started with MariaDB. Each of the SQL statements mentioned here have several more options and clauses each. We will cover these statements and others in greater detail in other articles. For now, though, you can learn more about them from experimenting and by further reading of the Knowledge Base online documentation. A downloadable PDF of much of the documentation is available here.