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var newsArticleQuery = Query.Where("SC_T_ID").IsEqualTo("-1234”); // only return news articles based on template ID

var newsQuery = newsArticleQuery; //this is so we can add a subquery

if (!String.IsNullOrEmpty(category))
    {
  	var categorySubQuery = SubQuery.Where("Md_Category").IsEqualTo(category);
	newsQuery = newsQuery.AndSubQuery(categorySubQuery );
    }

 var finalQuery = new NodeFinder().Find(newsQuery);