code: def emit_code_from_multiple_match_queries(match_queries):
      
      optional_variable_base_name = '$optional__'
      union_variable_name = '$result'
      query_data = deque([u'SELECT EXPAND(', union_variable_name, u')', u' LET '])
  
      optional_variables = []
      sub_queries = [emit_code_from_single_match_query(match_query)
                     for match_query in match_queries]
      for (i, sub_query) in enumerate(sub_queries):
          variable_name = optional_variable_base_name + str(i)
          variable_assignment = variable_name + u' = ('
          sub_query_end = u'),'
          query_data.append(variable_assignment)
          query_data.append(sub_query)
          query_data.append(sub_query_end)
          optional_variables.append(variable_name)
  
      query_data.append(union_variable_name)
      query_data.append(u' = UNIONALL(')
      query_data.append(u', '.join(optional_variables))
      query_data.append(u')')
  
      return u' '.join(query_data)
docs: Return a MATCH query string from a list of MatchQuery namedtuples.