select 1 from 浅析

今天看到项目代码里有这条语句，不懂select 1 from XXXXXXX里的1是何意，查了一番才知道：

1、select 1 from mytable;与select anycol(目的表集合中的任意一行） from mytable;与select * from mytable 作用上来说是没有差别的，都是查看是否有记录，一般是作条件用的。select 1 from 中的1是一常量，查到的所有行的值都是它，但从效率上来说，1>anycol>*，因为不用查字典表。

2、查看记录条数可以用select sum(1) from mytable;等价于select sum(*) from mytable;

3、实际项目SQL：

select c_insrnc_cde, c_nme_cn
   from t_prd_ins a
where a.c_prod_no = '01'
    and not exists
(select 1
           from (select d.c_insrnc_cde, a.c_nme_cn
                   from t_prd_ins                   a,
                        tb_bas_ply_policy_rdr       c,
                        tb_bas_ply_policy_rdr_list d
                  where a.c_insrnc_cde = d.c_insrnc_cde
                    and d.c_policy_id = c.c_policy_id
                    and d.c_seq_no = c.c_seq_no
                    and a.c_kind_no = '01'
                    and c.c_dpt_cde = '101'   ----机构树中取出机构
                    and c.c_prod_no = '01'
                    and c.t_effc_tm <= TO_DATE('2007-9-5', 'YYYY-MM-DD')
                    and c.t_expd_tm >= TO_DATE('2007-9-5', 'YYYY-MM-DD')
                    and c.c_work_mode = '00501'
                    and c.c_reach_area = '00701'
                    ) t
          where t.c_insrnc_cde = a.c_insrnc_cde);