Nagios is a powerful  monitoring tool for your server and you Know immediately on the browser when problems arise. You will need to configure a part of mysql monitoring in easy way  follow the below steps,

Client side Configuration:

By default check_mysql script located under /usr/local/nagios/libexec directory, before configure the check_mysql need to create a nagios user and give full permissions.

Open MySQL on terminal and execute the below queries.

# mysql    

mysql> CREATE USER 'nagios'@'%' IDENTIFIED BY 'nagios';
Query OK, 0 rows affected (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

mysql> exit;

Execute the below command for testing,
# /usr/local/nagios/libexec/check_mysql -u nagios -p nagios -H  -P 3306

Uptime: 873000  Threads: 10  Questions: 25999491  Slow queries: 34692  Opens: 939970  Flush tables: 1  Open tables: 400  Queries per second avg: 29.781|Connections=645881c;;; Open_files=708;;; Open_tables=400;;; Qcache_free_memory=0;;; Qcache_hits=0c;;; Qcache_inserts=0c;;; Qcache_lowmem_prunes=0c;;; Qcache_not_cached=0c;;; Qcache_queries_in_cache=0;;; Queries=25999491c;;; Questions=25996858c;;; Table_locks_waited=2762c;;; Threads_connected=7;;; Threads_running=3;;; Uptime=873000c;;;

u - User Name
p - Password
H - Host Name
P - Port Number

You may get an above output can add the command in nrep.cfg file /usr/local/nagios/etc/nrpe.cfg
command[check_mysql]=/usr/local/nagios/libexec/check_mysql -u nagios -p nagios -H   -P 3306

Server side Configuration:

Enter services to be monitored in host configuration file.
# vi  /usr/local/nagios/etc/objects/host.hostname.cfg

define service{
        use                          generic-service
        service_description   MYSQL
        check_command      check_nrpe!check_mysql

You will need to verify before starting nagios using command,
#  /usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg
Total Warnings: 0
Total Errors:   0
Things look okay - No serious problems were detected during the pre-flight check

Restart the Nagios server.
# /etc/init.d/nagios restart