code: public SQLUnit toSQL() {
          StringBuilder result = new StringBuilder();
          List<Object> insertParameters = new LinkedList<>();
          for (Object each : segments) {
              if (!(each instanceof ShardingPlaceholder)) {
                  result.append(each);
                  continue;
              }
              if (each instanceof InsertValuesPlaceholder) {
                  appendInsertValuesPlaceholder(null, (InsertValuesPlaceholder) each, insertParameters, result);
              } else if (each instanceof InsertSetPlaceholder) {
                  appendInsertSetPlaceholder(null, (InsertSetPlaceholder) each, insertParameters, result);
              } else {
                  result.append(each);
              }
          }
          return insertParameters.isEmpty() ? new SQLUnit(result.toString(), parameters) : new SQLUnit(result.toString(), insertParameters);
      }
docs: Convert to SQL unit.
  
  @return SQL unit