MySQL/Guide

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This is an introduction to MySQL and its use of databases, tables and privileges.

The document is based on the input from Gentoo's MySQL Startup Guide.

Setting up the database

Creating A Database

When you have logged in and a mysql prompt is displayed, let's then take a look at the databases we currently have. To do so, we use the SHOW DATABASES command.

user $mysql -u root -q
Enter password:                                                                              
Welcome to the MySQL monitor.  Commands end with ; or \g.                                    
Your MySQL connection id is 673                                                              
Server version: 5.1.62-log Gentoo Linux mysql-5.1.62-r1                                      
                                                                                             
Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.                 
                                                                                             
Oracle is a registered trademark of Oracle Corporation and/or its                            
affiliates. Other names may be trademarks of their respective                                
owners.                                                                                      
                                                                                             
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.               
                                                                                             
mysql>
mysql>SHOW DATABASES;
+----------+
| Database |
+----------+
| mysql    |
| test     |
+----------+
2 rows in set (0.09 sec)
Important
Please remember that MySQL commands should end with a semicolon (;)

Despite the fact that a test database is already created, we are going to create our own. Databases are created using the CREATE DATABASE command. We'll create one named "gentoo".

mysql>CREATE DATABASE gentoo;
Query OK, 1 row affected (0.08 sec)

The response lets us know that the command was executed without any errors. In this case, 1 row was modified. This is a reference to the main mysql database, which carries a list of all the databases. You won't need to worry too much about the background details. The last number is how fast the query was executed. We can verify the database was created by running the SHOW DATABASES command again.

mysql>SHOW DATABASES;
+----------+
| Database |
+----------+
| gentoo   |
| mysql    |
| test     |
+----------+
3 rows in set (0.00 sec)

Indeed our database has been created. In order to work with creating tables for our new gentoo database, we need to select it as our current database. To do so, we use the USE command. The USE command takes the name of the database you wish to use as your current database. Another option is to set it on the command line after the -D switch. Let's go ahead and switch to the gentoo database.

mysql>USE gentoo;
Database changed

And the current database is now our previously created gentoo database. Now that we're using it, we can start to create some tables and put information in them.

Working With Tables In MySQL

Creating a Table

In the structure of MySQL, there are databases, tables, records, and fields. Databases hold together tables, tables hold together records, records hold together fields, which contain the actual information. This structure lets users select how they want to access their information. So far we've dealt with databases, now let's work with tables. First off, tables can be listed similiarly to databases using the SHOW TABLES command. Right now there are no tables in our gentoo database, as running the command will show us:

mysql>SHOW TABLES;
Empty set (0.00 sec)

This means we need to create some tables. In order to do so, we use the CREATE TABLE command. However, this command is quite different from simple 'CREATE DATABASE command. This command takes a list of arguments. The form is as follows:

mysql>CREATE TABLE [table_name] ([field_name] [field_data_type]([size]);

table_name is the name of the table we wish to create. In this case, let's make a table named developers. This table will contain the developer's name, email and job. field_name will contain the name of the field. We have 3 required names in this case: name, email, and job. Also, a Primary Key field is defined for easy relation with other tables and will auto-increment with each insert. The Primary Key is recommended, but not required as MySQL will keep one internally. Indexes, of which the Primary Key belongs to, allow for faster data access as the tables grow. The field_data_type is what type of information will be stored. The different formats available can be found at the MySQL Column Types Page. For our purposes, we'll use the VARCHAR data type for all of our data fields and INTEGER for the key field. VARCHAR is one of the simplest of data types when it comes to working with strings. size is how much of data a single field will store. In this case, we'll use 128. This means that the field can have VARCHAR data that is 128 bytes. You can safely think of this as 128 characters for the time being, though there is a somewhat more technical explanation that the above site will provide you with. Now that we know how we are going to create the table, let's do it.

mysql>CREATE TABLE developers ( id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, name VARCHAR(128), email VARCHAR(128), job VARCHAR(128));
Query OK, 0 rows affected (0.11 sec)

Looks like our table was created OK. Let's check it with the SHOW TABLES command:

mysql>SHOW TABLES;
+------------------+
| Tables_in_gentoo |
+------------------+
| developers       |
+------------------+
1 row in set (0.00 sec)

Yes, there's our table. However, it doesn't seem to have any information on the types of fields we setup. For that, we use the DESCRIBE command (or DESC for short), which takes the name of the table as its argument. Let's see what that gives us for our developers table.

mysql>DESCRIBE developers;
+-------+--------------+------+-----+---------+----------------+
| Field | Type         | Null | Key | Default | Extra          |
+-------+--------------+------+-----+---------+----------------+
| id    | int(11)      | NO   | PRI | NULL    | auto_increment |
| name  | varchar(128) | YES  |     | NULL    |                |
| email | varchar(128) | YES  |     | NULL    |                |
| job   | varchar(128) | YES  |     | NULL    |                |
+-------+--------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

This shows the different fields and their types. It also shows a few extra attributes that are beyond the scope of this how-to. Feel free to refer to the MySQL Reference Manual for more information. We now have a table to work with. Now let's go ahead and populate it.

Populating Our MySQL Database

We populate a table (or add data) using the INSERT command. Like the CREATE TABLE command, it also has a specific format:

mysql>INSERT INTO table (col1, col2, ...) VALUES('value1', 'value2', ...);

This command is used to insert a record into table. table contains the MySQL table we wish to enter the information into. The table name may be followed by the list of columns to insert data into and VALUES() contains the values you wish to insert into the table. You may omit the list of columns if you insert a value into each one and if you write the values in the same order the columns have been defined. For the auto-increment column, a NULL value will use the next available value. In this case, we want to insert data into the developers table. Let's insert sample records:

mysql>INSERT INTO developers VALUES(NULL, 'Joe Smith', 'joesmith@gentoo.org', 'toolchain');
Query OK, 1 row affected (0.06 sec)

If you don't know the order of the columns in the table or want to insert an incomplete record:

mysql>INSERT INTO developers (job, name) VALUES('outsourced', 'Jane Doe');
Query OK, 1 row affected (0.01 sec)

According to our return result, it appears that the record was inserted correctly. What if we want to input more information than just one record? That's where the LOAD DATA command comes into play. This loads records from a tab separated file. Let's try that by editing a file in our home directory with the records. We'll call this file records.txt. Here's a sample:

CODE
3	John Doe	johndoe@gentoo.org	portage
4	Chris White	chriswhite@gentoo.org	documentation
5	Sam Smith	samsmith@gentoo.org	amd64
Important
Be sure you know what data you're dealing with. It's very unsafe to use LOAD DATA when you are uncertain of the file's contents!

Now the LOAD DATA command has a somewhat elongated definition, but we'll use the simplest form here.

mysql>LOAD DATA LOCAL INFILE '/path/to/filename' INTO TABLE table;

/path/to/filename is the directory and filename that will be used. table is the name of our table. In this case, our file is ~/records.txt and the table is developers.

mysql>LOAD DATA LOCAL INFILE '~/records.txt' INTO TABLE developers;
Query OK, 3 rows affected (0.00 sec)
Records: 3  Deleted: 0  Skipped: 0  Warnings: 0
Important
If you come up with any strange behavior, make sure your fields are separated by tabs. If you paste information into your infile from another source, it may convert your tabs to spaces.

That worked well. However, this simply inserts records, and does not give you any sort of control over MySQL. Many web applications use sql scripts in order to setup MySQL quickly and easily. If you want to use an sql script, you'll need to run mysql in batch mode, or source the file. Here's an example of running mysql in batch mode:

user $mysql -u root -h localhost -p < sqlfile

Like LOAD DATA, be sure you can tell what sqlfile does. Failure to do so may cause your database to be compromised! Another way you can accomplish this is to use the source command. This command will run mysql commands from an sql file while in the mysql interactive mode. Here is how to source an sql file:

mysql>source sqlfile;

If you see a web application wanting you to run an sql file, the two above commands can be used to accomplish that task. We have our table setup, so how do we check our fields? We do this by searching our table with queries.

Browsing MySQL Tables With Queries

Queries are one of the main features of any SQL database. They help us turn data in our tables into something useful. Most queries are done with the SELECT command. The SELECT command is fairly complex, and we're only going to look at three main forms of the command in this document.

(Select all entries in a table)

mysql>SELECT * FROM table;

(Select specific entries in a table)

mysql>SELECT * FROM table WHERE field=value;

(Select specific fields)

mysql>SELECT field1,field2,field3 FROM table [WHERE field=value];

Let's take a quick look at the first form. It's relatively simple and gives you an overall look of your table. We'll go ahead and run it to see what data we have so far.

mysql>SELECT * FROM developers;
+----+-------------+-----------------------+----------------+
| id | name        | email                 | job            |
+----+-------------+-----------------------+----------------+
|  1 | Joe Smith   | joesmith@gentoo.org   | toolchain      |
|  2 | Jane Doe    | NULL                  | Outsourced job |
|  3 | John Doe    | johndoe@gentoo.org    | portage        |
|  4 | Chris White | chriswhite@gentoo.org | documentation  |
|  5 | Sam Smith   | samsmith@gentoo.org   | amd64          |
+----+-------------+-----------------------+----------------+
5 rows in set (0.00 sec)

We see both the data we inserted through INSERT and those inserted by LOAD DATA present. Now, let's say that we just want to see the record for Chris White. We can do so with the second form of select as shown below.

mysql>SELECT * FROM developers WHERE name = 'Chris White';
+----+-------------+-----------------------+---------------+
| id | name        | email                 | job           |
+----+-------------+-----------------------+---------------+
|  4 | Chris White | chriswhite@gentoo.org | documentation |
+----+-------------+-----------------------+---------------+
1 row in set (0.08 sec)

As expected, the specific entry that we were looking for has been selected. Now, let's say we only wanted to know the person's job and email address, not their name. We can do so with the third form of SELECT as shown here.

mysql>SELECT email,job FROM developers WHERE name = 'Chris White';
+-----------------------+---------------+
| email                 | job           |
+-----------------------+---------------+
| chriswhite@gentoo.org | documentation |
+-----------------------+---------------+
1 row in set (0.04 sec)

This method of selection is a lot easier to manage, especially with larger amounts of information, as we'll see later on. Right now, being the root mysql user, we have unlimited permissions to do what we wish with the MySQL database. In a server environment, a user with such privileges can be quite problematic. In order to control who does what with the databases, we setup privileges.

MySQL Privileges

Privileges are what kind of access users have to databases, tables, pretty much anything. Right now in the gentoo database, the MySQL root account is the only account that can access it, given its permissions. Now, let's create two somewhat generic users, guest and admin, who will access the gentoo database and work with the information in it. The guest account will be a restricted one. All it will be able to do is get information from the database, and that's it. admin will have the same control as root, but only for the gentoo database (not the main mysql databases).

Creating Users

The CREATE USER SQL statement will define users and set the authentication method, commonly by password but other plugins may be available.

An example CREATE USER command is:

mysql>CREATE USER '[user]'@'[host]' IDENTIFIED BY '[password]';

To create our users for this example:

(admin)

mysql>CREATE USER 'admin'@'localhost' IDENTIFIED BY 'password';

(guest)

mysql>CREATE USER 'guest'@'localhost' IDENTIFIED BY 'password';
Important
A hostname of 'localhost' does not mean DNS localhost (127.0.0.1) to MySQL. Instead, it refers to the UNIX socket connection and not TCP/IP.

Granting Privileges with GRANT

Let's have a closer look at this somewhat simplified format of the GRANT command.

mysql>GRANT [privileges] ON database.* TO '[user]'@'[host]';

First we have the privileges we wish to assign. With what we've learned so far, here are some of the privileges you can set:

CODE
ALL - Gives the all privilege control for the database
    CREATE - Allows users to create tables
    SELECT - Allows users to query tables
    INSERT - Allows users to insert data into a table
    SHOW DATABASES - Allows users to see a list of databases
    USAGE - User has no privileges
    GRANT OPTION - Allows users to grant privileges
Note
If you're running MySQL to communicate data to a web application, CREATE, SELECT, INSERT (discussed here), DELETE and UPDATE (for further infomation look up the MySQL Reference Manual - GRANT and REVOKE Syntax section) are the only permissions you will most likely need. A lot of people make the mistake of granting all permissions when it's not really necessary. Check with the application developers to see if such permissions will cause issues with general operation.

For our admin user, ALL will do. For the guest user, SELECT will be sufficient for read only access. database is the database we wish the user to have these permissions on. In this example, gentoo is the database. The .* means all tables. If you wanted to, you could apply per table access. user is the name of the user and host is the hostname the user will be accessing from. In most cases, this will be localhost. Given the information, let's go ahead and give access to our users.

(admin)

mysql>GRANT ALL ON gentoo.* TO 'admin'@'localhost';

(guest)

mysql>GRANT SELECT ON gentoo.* TO 'guest'@'localhost';

Now that we have the users setup, let's test them out. First we quit mysql by typing quit at the command prompt:

mysql>quit

We're now back at the console. Now that we have our users setup, let's go ahead and see what they can do.

Testing User Permissions

We shall now attempt to login as the guest user. Currently, the guest user has SELECT only privileges. This basically comes down to being able to search and nothing more. Go ahead and login with the guest account.

user $mysql -u guest -h localhost -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 6 to server version: 4.0.25

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql>

Now we should test the user restriction(s). Let's switch to the gentoo database:

mysql>USE gentoo;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed

Now let's try to do something we are not supposed to. We'll attempt to create a table.

mysql>CREATE TABLE test (test VARCHAR(20), foobar VARCHAR(2));
ERROR 1044: Access denied for user: 'guest@localhost' to database 'gentoo'

As you can see, this function fails, as our user does not have the appropriate access. However, one access we did grant is the SELECT statement. Let's go ahead and try that:

mysql>SELECT * FROM developers;
+-------------+-----------------------+----------------+
| name        | email                 | job            |
+-------------+-----------------------+----------------+
| Joe Smith   | joesmith@gentoo.org   | toolchain      |
| John Doe    | johndoe@gentoo.org    | portage        |
| Chris White | chriswhite@gentoo.org | documentation  |
| Sam Smith   | samsmith@gentoo.org   | amd64          |
| Jane Doe    | NULL                  | Outsourced job |
+-------------+-----------------------+----------------+
5 rows in set (0.00 sec)

The command succeeds, and we're given a glimpse of what user permissions can do. We did, however, create an admin account as well. This was created to show that even all permissions granted users can still have limitations. Go ahead and quit MySQL and login as the admin.

mysql>quit
user $mysql -u admin -h localhost -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 7 to server version: 4.0.25

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql>

To begin, we'll try creating a new database with our admin user. This admin user will have access similiar to the root MySQL account, and will be able to do any kind of modification to the gentoo database it chooses. This will test the user's access to the main MySQL database. Remember ealier that we only set permissions to a specific database.

mysql>CREATE DATABASE gentoo2;
ERROR 1044: Access denied for user: 'admin@localhost' to database 'gentoo2'

Indeed, the admin user cannot create databases on the main MySQL database, despite all his permissions on the gentoo database. However, we're still able to use the admin account to modify the gentoo database, as shown here by this example data insertion.

mysql>USE gentoo;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql>INSERT INTO developers VALUES('Bob Simmons', 'bobsimmons@gentoo.org', 'python');
Query OK, 1 row affected (0.08 sec)

The admin user can access the database as they please. Now sometimes, we need to get rid of user permissions. This could be anything from a problematic user to a retired employee. Let's take a look at how to disable user permissions with the REVOKE command.

Removing User Access With The REVOKE Command

The REVOKE command lets us deny access to a user. We can either deny full access, or specific access. In fact, the format is very similiar to GRANT.

mysql>REVOKE [privileges] ON database.* FROM '[user]'@'[host]';

Options here are explained in the GRANT command section. In this section however, we're going to deny full access to a user. Let's say we find out the guest account is causing some problems security wise. We decide to revoke all privileges. We login as root and do the needful.

mysql>REVOKE ALL ON gentoo.* FROM 'guest'@'localhost';
Query OK, 0 rows affected (0.00 sec)
Note
In this case, user access is simple, so per database revoking is not a problem. However, in larger cases, you would most likely be using *.* instead of gentoo.* to remove user access to all other databases

Now let's quit and attempt to login as a guest user.

user $mysql -u guest -h localhost -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 9 to server version: 4.0.25

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql>

Although we're able to login, our access to gentoo is now gone.

mysql>USE gentoo;
ERROR 1044: Access denied for user: 'guest@localhost' to database 'gentoo'

And our problematic user is no longer able to access the gentoo database. Please note that the user was still able to login. That is because they remain in the main MySQL database. Let's take a look at how to completely remove an account with DELETE and the MySQL user table.

Removing Accounts

Make sure you're logged in as root.

mysql>DROP USER 'guest'@'localhost';
Query OK, 0 rows affected (0.00 sec)

It seems to have worked ok. Let's test by logging out and attempting to login as our guest user.

mysql>quit
user $mysql -u guest -h localhost -p
Enter password:
ERROR 1045: Access denied for user: 'guest@localhost' (Using password: YES)

Our user is now successfully deleted!

Interesting GUI tools

While this guide focuses mainly on setting up MySQL on the command line, a few alternatives are available in GUI form:

  • dev-db/phpmyadmin - Popular php based MySQL administration tool.
  • gmyclient - GNOME frontend.
  • mysqlnavigator - Qt frontend
  • knoda - KDE frontend.

This ends the MySQL First Steps tutorial. I hope this gives you a better understanding of the basics behind MySQL and getting a database set up.


This article is based on a document formerly found on our main website gentoo.org.
The following people contributed to the original document: Chris White (author), Shyam Mani (editor) and Xavier Neys (editor)
They are listed here as the Wiki history does not allow for any external attribution. If you edit the Wiki article, please do not add yourself here; your contributions are recorded on the history page.