Use of COALESCE in the SQL Server

DECLARE @RegistrationNo VARCHAR(1000);    

                    SELECT  @RegistrationNo = COALESCE( CASE WHEN CAST(@RegistrationNo AS VARCHAR(1000)) IS NOT NULL AND @RegistrationNo<>'' then CAST(@RegistrationNo AS VARCHAR(1000)) + ', ' end, '') +''+ CAST(RegistrationNo AS VARCHAR(100))   

                    FROM Table_Name

                    SELECT '[' + @RegistrationNo + ']'