Databases and MySQL
Contents
SQL With MySQL
SQL (Structured Query Language) is a language used to interact with a relational database. While SQL is the standard basis for database communication, different database vendors usually add their own small modifications or enhancements. In order to use SQL, you need to use a client that communicates with the server. Many high-level languages like Java, C, and PHP have the libraries for that purpose. Alternatively, you can use the command line program mysql to communicate with the server.
Installing MySQL
You will need both the mysql server and the mysql client. Installing the server automatically installs the client package, so you get them by running:
apt-get install mysql-server
When the server is being installed, it will ask you for a root password for the mysql server. Don't leave it blank! Pick anything you like for the password, but make sure you can remember it. You will need this password when you access your database server to administrate it. If you forget the password, you can reset it later, but it's not a simple process.
There are also several tools that help you manage mysql databases easily. One in particular is is PhpMyAdmin that allows you to manage your database over the web. You can install it with:
apt-get install phpmyadmin
Installing phpmyadmin will automatically configure Apache for you if you let it. When installing, select Apache2 when it asks which databases to auto configure. You will have to supply the root mysql password you chose when installing mysql. You will also have to add a PhpMyAdmin password for use later. Make sure to pick something you can remember.
Using MySQL
Starting MySQL
The interface to mysql is mysql command. This command invoked typically with the following format
mysql -u USERNAME -pPASSORD -hHOST databasename
Note that, the user needs to have access to the database otherwise, MySQL will prompt an error. See the next section to learn how to give users access to databases
Managing Users
The users in a MySQL database are stored in the "user" table of the "mysql" database. You can add a user by adding a row to the user table (see SQL commands in later sections). You can access the mysql database at localhost with:
mysql -u root -pyourpassword mysql
The fields of this table are:
+-----------------------+-----------------------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------------------+-----------------------------------+------+-----+---------+-------+
| Host | char(60) | NO | PRI | | |
| User | char(16) | NO | PRI | | |
| Password | char(41) | NO | | | |
| Select_priv | enum('N','Y') | NO | | N | |
| Insert_priv | enum('N','Y') | NO | | N | |
| Update_priv | enum('N','Y') | NO | | N | |
| Delete_priv | enum('N','Y') | NO | | N | |
| Create_priv | enum('N','Y') | NO | | N | |
| Drop_priv | enum('N','Y') | NO | | N | |
| Reload_priv | enum('N','Y') | NO | | N | |
| Shutdown_priv | enum('N','Y') | NO | | N | |
| Process_priv | enum('N','Y') | NO | | N | |
| File_priv | enum('N','Y') | NO | | N | |
| Grant_priv | enum('N','Y') | NO | | N | |
| References_priv | enum('N','Y') | NO | | N | |
| Index_priv | enum('N','Y') | NO | | N | |
| Alter_priv | enum('N','Y') | NO | | N | |
| Show_db_priv | enum('N','Y') | NO | | N | |
| Super_priv | enum('N','Y') | NO | | N | |
| Create_tmp_table_priv | enum('N','Y') | NO | | N | |
| Lock_tables_priv | enum('N','Y') | NO | | N | |
| Execute_priv | enum('N','Y') | NO | | N | |
| Repl_slave_priv | enum('N','Y') | NO | | N | |
| Repl_client_priv | enum('N','Y') | NO | | N | |
| Create_view_priv | enum('N','Y') | NO | | N | |
| Show_view_priv | enum('N','Y') | NO | | N | |
| Create_routine_priv | enum('N','Y') | NO | | N | |
| Alter_routine_priv | enum('N','Y') | NO | | N | |
| Create_user_priv | enum('N','Y') | NO | | N | |
| ssl_type | enum('','ANY','X509','SPECIFIED') | NO | | | |
| ssl_cipher | blob | NO | | | |
| x509_issuer | blob | NO | | | |
| x509_subject | blob | NO | | | |
| max_questions | int(11) unsigned | NO | | 0 | |
| max_updates | int(11) unsigned | NO | | 0 | |
| max_connections | int(11) unsigned | NO | | 0 | |
| max_user_connections | int(11) unsigned | NO | | 0 | |
+-----------------------+-----------------------------------+------+-----+---------+-------+
Important fields are Host (specifies from which host a user can access the database), User (name of the user), Password (the password for this user), and several boolean privilege attributes. MySQL provides grant command that inserts a new row to this table with most of attributes set automatically. The format of grant is
grant PRIVILEGE_NAMES on DATABASE.TABLE to 'USERNAME'@'HOSTNAME' identified by 'USERPASSWORD';
The privileges could be individual privileges, such as SELECT, INSERT, DROP etc., or it could be a group of privileges (USAGE) or it could be ALL privileges. The table name could be a wild card (*), the hostname can be anyhost, or it could contain a wild card '%'. Please note that, if you have multiple client hosts to access this database, you would need to grant permissions for each host.
The following command gives all priviliges to the user boss' who has the password employer if the user connects from the localhost.
grant all on company.* to 'boss'@'localhost' identified by 'employer';
After each grant, you need to tell mysql server to reload grant tables by flushing privileges.
flush privileges;
Managing Databases
The databases in mysql are crated with
create database DATABASENAME;
command. You should have administrative rights to be able to do that. A database can be deleted with drop command:
drop database DATABASENAME;
In mysql command prompt, you can change your active database with
use DATABASE;
Once the active database is changed, all your command will be applied to the new active database.
For example, the following commands will create and switch to a new database
create database company; use company;
Managing Tables
Similar to databases, tables are created and deleted with create table, drop table command. When creating a table, the user needs to supply the attribute information.
create table TABLENAME (ATTRIBUTE TYPE, ATTRIBUTE TYPE,.....);
MySQL contains several attribute types for strings, numerical values, dates etc. The common types include char, varchar, text, blob, int, float,date etc. MySQL manual contains full list of available types [[1]]
For example, the following command will create a new table called employee
create table employee ( name char(20), dept char(20), jobtitle char(20));
You can see the tables in a database with
show tables;
command. You can get a detailed description of a table with:
describe TABLENAME;
SQL Commands
Inserting New Data
insert into command inserts a new row to the database
insert into TABLENAME values (ATTRIBUTE1_VALUE, ATTRIBUTE2_VALUE,.....)
If you want to insert a new row with only some attributes set, you can do that by telling which attributes you want to be set:
insert into TABLENAME (ATTIBUTE1,ATTRIBUTE2) values (ATTRIBUTE1_VALUE, ATTRIBUTE2_VALUE)
For example, the following commands will insert new employees to the company database.
insert into employee values ("Alice","Wonderland","Lost traveler");
insert into employee values ("Peter","Neverland","Leader");
insert into employee values ("Frodo","Shire","Ring beerer");
Retrieving Existing Data
select command returns the rows from a database where some conditions are matched.
select ATTRIBUTE1,ATTRIBUTE2,..... from TABLE [where CONDITION]
Instead of individual attributes, you can ask to receive all row with select *
For example
select * from employee;
will return
+-------+------------+---------------+
| name | dept | jobtitle |
+-------+------------+---------------+
| Alice | Wonderland | Lost traveler |
| Peter | Neverland | Leader |
| Frodo | Shire | Ring beerer |
+-------+------------+---------------+
or you can get Peter's title.
select `jobtitle` from employee where `name` like 'Peter';
Updating Existing Data
update command is used to update individual rows. The format is
update TABLENAME set ATTRIBUTE1=value1,ATTRIBUTE2=value2 .... [where conditions]
For example, the following will update Frodo's job title.
update employee set `jobtitle`='Hero' where `name` like 'Frodo'
Deleting Existing Data
delete from command is used to delete row(s) from the database
delete from employee where CONDITION;
For example, the following command will delete Frodo.
delete from employee where `name` like 'Frodo';
where as
delete from employee;
deletes everything from the table.
PHP and MySQL
In order to use mysql function in php, you need to install php5-mysql package first. Then from a PHP page, you can connect to a mysql database by using mysql_connect command.
$link=mysql_connect('HOSTNAME','DATABASEUSER','PASSWORD');
The hostname is either IP or name of the server running MySQL daemon, the database user is the name of the user who can run the required commands and password is the user's the user's password. mysql_connect returns a mysql object that you will use later to communicate with the server. If the connection fails, $link will be NULL. Once connected, you can select a database:
mysql_select_db('DATABASENAME',[$link]);
If $link is not specified, mysql_select_db tries to find an open connection.
You can close a connection with mysql_close
mysql_close($link);
SQL commands are send through mysql_query command.
mysql_query(SQLQUERY)
for example,
mysql_query("insert into employee values ('Sheridan','B5','Commander')") or die('Error, insert query failed');
will insert data to employee table. mysql_query also receive the data from the server. For example, if you have send a select query, then you can loop over the results for each returned row.
$result = mysql_query('select * from employee');
while($row = mysql_fetch_array($result, MYSQL_ASSOC))
{
echo "Name :{$row['name']} <br>" .
"Department : {$row['dept']} <br>" .
"Job Title : {$row['jobtitle']} <br><br>";
}
After the sql command is sent, the individual rows are fetched with mysql_fetch_array. The parameter MYSQL_ASSOC tells the function that the results should be in the format of an associative array, i.e., the attributes can be reached by their names. If you want a numeric array, you can use MYSQL_NUM. The default is MYSQL_BOTH, hence you can avoid specifying this parameter. If you are interested in the number of rows returned, you can call mysql_num_rows function.
PHP Manual has a very detailed description of each mysql related function [2]
Other MySQL Information
The configuration file for MySQL is /etc/mysql/my.cnf. It specifies MySQL directories and other parameters. If you want to migrate MySQL, it is sufficient to copy files from datadir, which is in debian located at /var/lib/mysql. One interesting parameter is bind-address. By default, it restricts the database access to the localhost, so you may want to comment it out.
The program mysqladmin is the official program to communicate with the server for managerial tasks. Common tasks that can be handled by mysqladmin include flushing some MySQL elements (such as tables, privileges etc.), kill mysql threads, restart, shutdown, or validate a client. In order to use this command, you need to provide the administrator password.
