code: def validate()
        if !@select.to_s.strip.empty? and @from.to_s.strip.empty?
          raise 'FROM clause is required with SELECT'
        end
        if !@from.to_s.strip.empty? and @select.to_s.strip.empty?
          raise 'SELECT clause is required with FROM'
        end
        if !@offset.nil? and @limit.nil?
          raise 'LIMIT clause is required with OFFSET'
        end
        unless @limit.is_a? Integer or @limit.nil?
          raise 'LIMIT must be an integer'
        end
        unless @offset.is_a? Integer or @offset.nil?
          raise 'OFFSET must be an integer'
        end
      end
docs: Return a list of validation errors.