Run a query based on variables passed

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C#

I am reading two different text files, one file reads the transaction ID and runs the query below to match the text file record. The other text file checks for Customer ID and runs this same query as well. What I need to do is build maybe a stringbuilder? So when it runs the Transaction textfile it elimates the "CustomerID IS NOT NULL" from the where clause.

What I have tried:

private DataRowCollection ExtractData(DateTime DailyFileDate)
       {
           Log.Info("Querying Microbilt data...");

           string sql = string.Empty;

           {
           sql = @"
                           SELECT
                               TransID,
                               CustomerID,
                               RID,
                               FileDate
                           FROM
                           (
                               SELECT TransID AS TransID,
                               CustomerID,
                               RID,
                               FileDate,
                               ROW_NUMBER() OVER(PARTITION BY TransID ORDER BY TransID DESC) AS RN
                               FROM MicroBilt.dbo.TransIDIgnoreAudit
                               WHERE FileDate BETWEEN @FileDate AND DATEADD(D,1, @FileDate AND CustomerID IS NOT NULL
                           ) A
                           WHERE RN = 1";