Update Certain Field In Database

I want to create a web page using ASP.NET (C#, SQL queries) to keep track the login and logout time of all employees of a small store and total hours they work every day. Here are the fields in my table (employID, logintime, logouttime, totalhours). Once an employee log in, he just clicks the Login/Logout button. The program will store the current system time into the "logintime" field while leaving the "logouttime" field and the "totalhours" field empty until he clicks the button again at the end of the day. So when an employee clicks the button, I want to determine if he wants to login or logout by examing the "logouttime" field. If it's emty (NULL), I know he wants to logout. If the "logouttime" field is not empty, I know he wants to login so my progam will create a new entry for him. My question is, what is the best way to determine if he wants to login or logout? This is how I approach it:I create a SELECT query: "SELECT COUNT(*) FROM mytable WHERE logoutime IS NULL and employID = @employeeID" (this employeeID is the ID an employee uses to log in the web page). I use this "count" value to determine if the next operation is to let him logout or create a new entry. If count # 0, that means the employee needs to logout. I just need an UPDATE command to update value for logouttime field. I need another "SELECT * FROM mytable WHERE logouttime is NULL and employID= @employeeID" because my previous SELECT query returns only the count value. I will use a DataReader for the 2nd SELECT to walk through the fileds of the matched row to retrieve the value of "logintime" field for subtracting operation to get the total hours. Is there a way I can access the table to determine if the employee "logouttime" field is empty and at the same time using that open connection to retrieve his information if the "logouttime" field is empty? I hope what i'm trying to say make sense. I haven't coded it yet. Just logics at this point.