In this article I would show you 2 simple examples of how to use column validation, at the column level, then at the list level.
One of the disadvantages when we work with SharePoint 2007 is the column validation but in 2010 it is fully resolved, ability to easily validate the column values entered into list items is a wonderful addition to SharePoint 2010. In this article I will show you 2 simple examples of how to use column validation; at the column level, then at the list level. Before starting on it let me introduce another good feature of SharePoint 2010. If you want to create a unique column in SharePoint 2010, it is very easy you don't have to write an event handler or features like we do in 2007. While creating the column just click on Enforce unique Values: "Yes". You are done!!Column Level Validations
SharePoint Online And Office 365 Administration