How to search through all the records in a database table in runtime?

Hi,

Any help on this would be great!

I currently have a database table (Blog) that holds usernames and I have a webform that allows users to input data into this table.

The table (blog) contains two attributes which are UserName & TitleBlog. The UserName is a primary key which does not allow duplicate values.

 However on the webform users can add their name UserName which if the same as a value in the database will bring up an error.

 What I want to be able to do is on the same web form, before a user inserts the new data (with the possible duplicate value) is to search the Blog Table and if the value in the Textbox (called TextboxUsername) is the same as a value in the database table to then throw up message saying to you cannot use this username.

 What I really am getting at is how do I search through each record in the table and compare it against a value in a textbox\label?

 Thanks f ...