Ref: http://www.nagiosexchange.org/cgi-bin/page.cgi?g=Detailed%2F1435.html;d=1

Written in Perl.  Requires FreeTDS to be install.

This plugin can query a Microsoft SQL Server or a MySQL Server. The plugin can also execute specific queries or stored procedures and return the results based. The results can then be compared via thresholds for numeric values or via regular expressions for string values.

Here is a good example of how I used the plugin in my blog.  Count Log Entries Stored Procedure and check by Nagios.