Update a c# project to return multiple id's as opposed to single id

The following is a web service to return a view from a sql database. I need to include an IN clause to allow for multiple groups (external ID) on the return query. This may need to be serialised to ensure that all the groups (external IDs) are returned. Here is a snapshot of the code below: [AccessKeyAuthorization] public class XmlViewsController : ApiController { public HttpResponseMessage Get(string accessKey, string viewName, string fieldName = null, string fieldValue = null) {if (accessKey == "293892380") { viewName = "ViewName1"; }; //To ensure only the relevant View is allowed string vToRedirect = "No"; switch(viewName) { case "ViewName2": vToRedirect = "Yes"; break; } if (vToRedirect == "No") { return new HttpResponseMessage([login to view URL]); }; string dataSetName = "DataSet"; string userIdFiledName = "Team"; var cn = new SqlConnection([login to view URL]["DataConnection"].ConnectionString); var cmd = new SqlCommand([login to view URL]("SELECT * FROM [{0}] WHERE 1=1", viewName), cn); var adapter = new SqlDataAdapter(cmd); var dataSet = new DataSet(dataSetName); [login to view URL](dataSet, [login to view URL], viewName); // filter by user id if ([login to view URL][viewName].[login to view URL](userIdFiledName)) { var userIdColumn = [login to view URL][viewName].Columns[userIdFiledName]; // TO SKIP THE FIELD SELECTION ON TEAM IF TEAM IS ALL if ((User as User).ExternalId == "ALL") { if ([login to view URL] == typeof(string)) { //[login to view URL] += [login to view URL](" AND [{0}]=@Team", userIdFiledName); //[login to view URL](new SqlParameter("@Team", "%")); }} // NEED TO CONVERT SO THAT IT IS NOT ONLY A SINGLE TEAM, BUT A LIST FROM A TABLE else { if ([login to view URL] == typeof(string)) { [login to view URL] += [login to view URL](" AND [{0}]=@Team", userIdFiledName); [login to view URL](new SqlParameter("@Team", (User as User).ExternalId)); }}} // filter by field value if ( &&  && [login to view URL][viewName].[login to view URL](fieldName)) {var fieldColumn = [login to view URL][viewName].Columns[fieldName]; [login to view URL] += [login to view URL](" AND [{0}]=@FieldValue", fieldName); [login to view URL](new SqlParameter("@FieldValue", [login to view URL](fieldValue, [login to view URL])));} var content = new HttpResponseMessage([login to view URL]) {Content = new StringContent([login to view URL](dataSetName, viewName, Url))}; [login to view URL] = new MediaTypeHeaderValue("application/xml"); return content; }}}