DataQuery - "Where" only for not NULL fields

Hello,I'm trying to build search functionality on my page. I've created form and results table. Everything works fine until I'm trying to search for more than one @.Where@ condition. How can I add dynamically another @.Where@ if session for field has some value and is not empty? That's my query in custom webpart: DataSet memberResults = new DataQuery(@store.member.allMembers@) .Columns(@store_member.firstName@, @store_member.lastName@, @store_member.emailAddress@, @store_member.displayName@) .Where(@store_member.firstName@, QueryOperator.Equals, firstName) //I want to add this another @.Where@ if @lastName@ field has some value //.Where(@store_member.lastName@, QueryOperator.Equals, lastName) .Execute();