Question

I have built a select statement to pick the date the participant was first recorded in the database. However I want to create a function out of this select statement to use it. I am fairly new to SQL and have never built a function before. My select statement looks like this:

Select DATEDIFF(day, (select min(startdatetime)
                        from GamePlay
                     ), enddatetime)
  from GamePlay
 where ParticipantID = '200'

My attempted function looks like this:

CREATE FUNCTION daysPlayed (@ParticipantID int)
RETURNS DateTime
AS
BEGIN
    Return DATEDIFF(day, (select min(startdatetime)
                            from GamePlay
                         ), enddatetime)
      from GamePlay
     where ParticipantID = @ParticipantID
END
GO

Answer 1

DATEDIFF does not return a datetime. It returns an int.

Your function might look like this:

CREATE FUNCTION daysPlayed (@ParticipantID int)
RETURNS INT
AS
BEGIN
    DECLARE @result INT
    SELECT @result=DATEDIFF(day, (select min(startdatetime) from GamePlay), enddatetime) from GamePlay where ParticipantID = @ParticipantID
    RETURN @result

END

but i don’t think it will do what you want it to do.
Also, please note that using functions in selects over a large number of rows is a guarantee performance hit and you might not be able to use some of the indexes you set up in your tables.