Where clause in sql
IF @StatusId = 10
    BEGIN
        SELECT
            *
        FROM
        Risk AS R
        INNER JOIN Statuses AS St ON R.Status_Id=St.Status_Id
        WHERE
        R.MitigationOwner = COALESCE(@MitigationOwner,R.MitigationOwner)
        AND R.RiskFactor = COALESCE(@RiskFactor,R.RiskFactor)
        AND R.RiskArea = COALESCE(@RiskArea,R.RiskArea)
        AND R.AddedWhen BETWEEN
        COALESCE(CONVERT(DATETIME, @StartDate+'00:00:00',120),R.AddedWhen) AND
        COALESCE(CONVERT(DATETIME,@EndDate+'23:59:59',120),R.AddedWhen)
    END

SELECT
            *
        FROM
        Risk AS R
        INNER JOIN Statuses AS St ON R.Status_Id=St.Status_Id
        WHERE
        (R.MitigationOwner = COALESCE(@MitigationOwner,R.MitigationOwner) OR R.MitigationOwner IS NULL)
        AND R.RiskFactor = COALESCE(@RiskFactor,R.RiskFactor)
        AND R.RiskArea = COALESCE(@RiskArea,R.RiskArea)
        AND (R.AddedWhen BETWEEN
        COALESCE(CONVERT(DATETIME, @StartDate+'00:00:00',120),R.AddedWhen) AND
        COALESCE(CONVERT(DATETIME,@EndDate+'23:59:59',120),R.AddedWhen) OR R.AddedWhen IS NULL)

...
(R.MitigationOwner = @MitigationOwner OR  @MitigationOwner IS NULL)
...

WHERE
R.MitigationOwner = COALESCE(@MitigationOwner,R.MitigationOwner)

WHERE
((R.MitigationOwner IS NULL AND @MitigationOwner IS NULL)
 OR (R.MitigationOwner = @MitigationOwner))