Example Doesn't Displaying Email Of Given User If Phone Is NULL In Db?

I'm trying to make filters on gridview. Gridview is bound to SqlDataSourceControl.

The problem is when there is NULL value in DB.

Parameters

Code:

[code]....

Select Statement

Code:

SELECT Name, Phone, email FROM suragch WHERE (firstname LIKE @firstname + '%') AND (phone LIKE @phone + '%') AND (email LIKE @email + '%')

Above example works fine if there is no NULL.

When users want to find email they leave phone field empty and enters the firstname. But my example doesn't displaying email of given user if phone is NULL in db.