Database

CREATE PROC sp_GetAvgRatings (@Role int)
AS
    SELECT
        U.[Name],
        AVG(R.Rating) As AvgRating
    
    FROM
        (
            -- Filter for role
            SELECT
                ID,
                [Name]
            FROM
                Users 
            WHERE
                (Role = @Role OR @Role Is Null)
        ) As U
        
        INNER JOIN
        (
            -- Get rid of null values
            SELECT
                PersonID,
                Rating
            FROM
                References
            WHERE
                NOT (Rating Is Null)
        ) As R
        
        ON U.ID = R.PersonID
        
    GROUP BY
        U.[Name]