Validate Data Entry in SharePoint 2010
Column validation options are new to SharePoint 2010 lists and allow you to define additional limits and constraints for your data. For example, you may want to ensure that a value in one Date column occurs after another Date column.
To use column validation on your list:
Click the List Settings button on the List tab.
Make sure you're in your list where you want to validate data entry.
Under General Settings, click the Validation Settings link.
An example of when to use column validation is to ensure that the date in the Date Finished column can’t be earlier than the date in the Date Started column — you can’t finish a project before it’s begun!
Type a formula in the Formula text box.
The result of the formula must evaluate to TRUE to pass validation. The formula syntax is the same as calculated columns, which is similar to Excel syntax.
Enter a user message that you want to appear when the validation formula fails.
The message should give the user an idea of how the formula works and how to fix the problem.
Click the Save button.
When users enter data into your form, the validation formula is evaluated. If the formula evaluates to FALSE, your user message appears on the form.
You can add column validation to columns created at the list or site level. Validation created for site-level columns applies everywhere that column is used, although the formula can be overridden at the list where the site-level column is used.