Can anyone suggest me which database to be used with OpenLayers? I have to store the values in a database table and retrieve them (trivial tasks). Which is better MySQL or PostGIS? Can anybody give benefits of these?
The bostongis comparison is not a really up-to-date document but it can be a good start, at least to see which aspects is a priority to you, and what you should consider as mandatory or optional to make your choices. The comparison is generic, not related to openlayers.
For your OL project, I guess you should also consider the DB availability from your hosting unless you have no constraint regarding this point.
From Postgis, you can natively export a query result as kml and json (and easily read it with OL), which is I think impossible with MySQL (you'll have a third party call to dynamically create it through any language..)
Postgres with PostGIS everytime http://en.wikipedia.org/wiki/PostGIS
More Spatial Functions and control on the database in spatial terms
If we are talking about Spatial Database, I would leave MySQL out: it is very much incomplete in respect with other OGC Simple Features SQL implementations like PostGIS (my favourite), Oracle Spatial, SQL Server Spatial or even Spatialite. Or ArcSde as a gateway to its supported RDBMS.
Please note, however, that you do not need to have a spatial database for using OpenLayers. You can feed OpenLayers with any Map Server that provides WFS/WMS/WCS etc... OGC web services implementations. For example if you are using MapServer, you will be able to feed OpenLayer with any vector layer supported by GDAL/OGR:
Given your use case ("store the values in a DB table and retrieve them") I guess you need a WFS-T implementations: you may use GeoServer (with PostGis/Oracle Spatial or MySql) or TinyWFS (only GeoServer) for this.
Note that TinyOWS is going to be included soon in the MapServer project (actually MapServer does not provide WFS-T implementation).