今天看到項(xiàng)目代碼里有這條語句，不懂select 1 from XXXXXXX里的1是何意，查了一番才知道：

1、select 1 from mytable;與select anycol(目的表集合中的任意一行） from mytable;與select * from mytable 作用上來說是沒有差別的，都是查看是否有記錄，一般是作條件用的。select 1 from 中的1是一常量，查到的所有行的值都是它，但從效率上來說，1>anycol>*，因?yàn)椴挥貌樽值浔怼?/P>

2、查看記錄條數(shù)可以用select sum(1) from mytable;等價(jià)于select sum(*) from mytable;

3、實(shí)際項(xiàng)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'   ----機(jī)構(gòu)樹中取出機(jī)構(gòu)
                    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);