SQL Query with optional WHERE clause parameter for date range? I have a procedure that takes, among other things,  start and end date parameters. These two parameters are optional inputs and default to NULL. What I would like to do is modify one of the current SELECT statements so that if the start and end dates are supplied it will return only the results between those two dates. I considered just throwing a CASE WHEN... in the WHERE statement but that that is messy and does not seem right. Is that the appropriate way to deal with this or is there something more efficient?

Existing SELECT clause:

SELECT  do.OrdrNmbr AS 'Order Number',
        mb.BOLNmbr AS 'BOL Number',
        do.DlvryDt  AS 'Deliver Date',
        mh.Text3 AS 'Truck',
        do.DlvrdQntty AS 'Gallons Delivered',
        ba.BANme AS 'Carier Business Associate'
FROM dbo.DeliveryOrder AS do WITH (NOLOCK)
INNER JOIN dbo.ManifestBOL AS mb WITH (NOLOCK)
    ON do.OrdrNmbr = mb.OrdrNmbr
INNER JOIN dbo.BusinessAssociate AS ba WITH (NOLOCK)
    ON do.CrrrBAID = ba.BAID
INNER JOIN SRA.dbo.MovementDocument as md WITH (NOLOCK)
    ON md.MvtDcmntExtrnlDcmntNbr = 'DOD' + CONVERT(VARCHAR, mb.OrdrNmbr)
INNER JOIN SRA.dbo.MovementHeader AS mh WITH (NOLOCK)
   ON mh.MvtHdrMvtDcmntID = md.MvtDcmntID
WHERE mb.OrdrNmbr = @OrderNumber;


and the start/end dates would run against do.DlvryDt.

P.S. I know the table/column names are horrid but this is a 3rd party application the query is running against.
Assuming that your start date and end date are compared against DlvryDt in DeliveryOrder table, I suggest to use a very low date value for start date and a very high date value for end date in cases where these date variables are null. Please see following query. 

SELECT  do.OrdrNmbr AS 'Order Number',
        mb.BOLNmbr AS 'BOL Number',
        do.DlvryDt  AS 'Deliver Date',
        mh.Text3 AS 'Truck',
        do.DlvrdQntty AS 'Gallons Delivered',
        ba.BANme AS 'Carier Business Associate'
FROM dbo.DeliveryOrder AS do WITH (NOLOCK)
INNER JOIN dbo.ManifestBOL AS mb WITH (NOLOCK)
    ON do.OrdrNmbr = mb.OrdrNmbr
INNER JOIN dbo.BusinessAssociate AS ba WITH (NOLOCK)
    ON do.CrrrBAID = ba.BAID
INNER JOIN SRA.dbo.MovementDocument as md WITH (NOLOCK)
    ON md.MvtDcmntExtrnlDcmntNbr = 'DOD' + CONVERT(VARCHAR, mb.OrdrNmbr)
INNER JOIN SRA.dbo.MovementHeader AS mh WITH (NOLOCK)
   ON mh.MvtHdrMvtDcmntID = md.MvtDcmntID
WHERE mb.OrdrNmbr = @OrderNumber
and do.DlvryDt &gt;= ISNULL(@StartDate, '19000101')
and do.DlvryDt &lt; ISNULL(dateadd(day, 1, @EndDate), '21000101');