Как вывести строки только с уникальным значением?

/* Optimize distinct away if possible */
  {
    ORDER *org_order= order;
    order=remove_const(this, order,conds,1, &simple_order);
    if (thd->is_error())
    {
      error= 1;
      DBUG_PRINT("error",("Error from remove_const"));
      DBUG_RETURN(1);
    }

    /*
      If we are using ORDER BY NULL or ORDER BY const_expression,
      return result in any order (even if we are using a GROUP BY)
    */
    if (!order && org_order)
      skip_sort_order= 1;
  }
  /*
     Check if we can optimize away GROUP BY/DISTINCT.
     We can do that if there are no aggregate functions, the
     fields in DISTINCT clause (if present) and/or columns in GROUP BY
     (if present) contain direct references to all key parts of
     an unique index (in whatever order) and if the key parts of the
     unique index cannot contain NULLs.
     Note that the unique keys for DISTINCT and GROUP BY should not
     be the same (as long as they are unique).

     The FROM clause must contain a single non-constant table.
  */
  if (tables - const_tables == 1 && (group_list || select_distinct) &&
      !tmp_table_param.sum_func_count &&
      (!join_tab[const_tables].select ||
       !join_tab[const_tables].select->quick ||
       join_tab[const_tables].select->quick->get_type() !=
       QUICK_SELECT_I::QS_TYPE_GROUP_MIN_MAX))
  {
    if (group_list && rollup.state == ROLLUP::STATE_NONE &&
       list_contains_unique_index(join_tab[const_tables].table,
                                 find_field_in_order_list,
                                 (void *) group_list))
    {
      /*
        We have found that grouping can be removed since groups correspond to
        only one row anyway, but we still have to guarantee correct result
        order. The line below effectively rewrites the query from GROUP BY
        <fields> to ORDER BY <fields>. There are two exceptions:
        - if skip_sort_order is set (see above), then we can simply skip
          GROUP BY;
        - we can only rewrite ORDER BY if the ORDER BY fields are 'compatible'
          with the GROUP BY ones, i.e. either one is a prefix of another.
          We only check if the ORDER BY is a prefix of GROUP BY. In this case
          test_if_subpart() copies the ASC/DESC attributes from the original
          ORDER BY fields.
          If GROUP BY is a prefix of ORDER BY, then it is safe to leave
          'order' as is.
       */
      if (!order || test_if_subpart(group_list, order))
          order= skip_sort_order ? 0 : group_list;
      /*
        If we have an IGNORE INDEX FOR GROUP BY(fields) clause, this must be
        rewritten to IGNORE INDEX FOR ORDER BY(fields).
      */
      join_tab->table->keys_in_use_for_order_by=
        join_tab->table->keys_in_use_for_group_by;
      group_list= 0;
      group= 0;
    }
    if (select_distinct &&
       list_contains_unique_index(join_tab[const_tables].table,
                                 find_field_in_item_list,
                                 (void *) &fields_list))
    {
      select_distinct= 0;
    }
  }
  if (group_list || tmp_table_param.sum_func_count)
  {
    if (! hidden_group_fields && rollup.state == ROLLUP::STATE_NONE)
      select_distinct=0;
  }
  else if (select_distinct && tables - const_tables == 1 &&
           rollup.state == ROLLUP::STATE_NONE)
  {
    /*
      We are only using one table. In this case we change DISTINCT to a
      GROUP BY query if:
      - The GROUP BY can be done through indexes (no sort) and the ORDER
        BY only uses selected fields.
    (In this case we can later optimize away GROUP BY and ORDER BY)
      - We are scanning the whole table without LIMIT
        This can happen if:
        - We are using CALC_FOUND_ROWS
        - We are using an ORDER BY that can't be optimized away.

      We don't want to use this optimization when we are using LIMIT
      because in this case we can just create a temporary table that
      holds LIMIT rows and stop when this table is full.
    */
    JOIN_TAB *tab= &join_tab[const_tables];
    bool all_order_fields_used;
    if (order)
      skip_sort_order= test_if_skip_sort_order(tab, order, select_limit, 1,
        &tab->table->keys_in_use_for_order_by);
    if ((group_list=create_distinct_group(thd, select_lex->ref_pointer_array,
                                          order, fields_list, all_fields,
                          &all_order_fields_used)))
    {
      bool skip_group= (skip_sort_order &&
        test_if_skip_sort_order(tab, group_list, select_limit, 1,
                                &tab->table->keys_in_use_for_group_by) != 0);
      count_field_types(select_lex, &tmp_table_param, all_fields, 0);
      if ((skip_group && all_order_fields_used) ||
      select_limit == HA_POS_ERROR ||
      (order && !skip_sort_order))
      {
    /*  Change DISTINCT to GROUP BY */
    select_distinct= 0;
    no_order= !order;
    if (all_order_fields_used)
    {
      if (order && skip_sort_order)
      {
        /*
          Force MySQL to read the table in sorted order to get result in
          ORDER BY order.
        */
        tmp_table_param.quick_group=0;
      }
      order=0;
        }
    group=1;                // For end_write_group
      }
      else
    group_list= 0;
    }
    else if (thd->is_fatal_error)           // End of memory
      DBUG_RETURN(1);
  }