select  (select count(account_id) from 
            (select account_id 
             from tab
             where substr(visit_time,0,10) = '2012-02-15'
             group by account_id
             having count(account_id)=1)) o_num,
         (select count(0) from tab where substr(visit_time,0,10) = '2012-02-15') t_num
from dual

------解决方案--------------------
SQL code
select sum(decode(rcnt,1,1,0)), 
       sum(rcnt)
from(select account_id,
     max(rn) rcnt 
     from(select account_id, 
                 row_number()over(partition by account_id order by id ) rn 
          from tab 
          where to_char(visit_time,'YYYY-MM-DD')='2012-02-15')
     group by account_id)

------解决方案--------------------
SQL code

select (select count(account_id) from 
              (select account_id from tab 
               where substr(visit_time,0,10)='2012-02-15'
               group by account_id
               having count(account_id)=1)) o_num,
       (select count(0) from tab where substr(visit_time,0,10)='2012-02-15') t_num
from dual

------解决方案--------------------

SQL code

SELECT sum(CASE WHEN num=1 THEN 1 ELSE 0 END) Login_Once_Count,sum(num) Login_All_Count
FROM 
(
SELECT account_id,count(account_id) num FROM tab 
WHERE trunc(visit_time)=to_date('15-FEB-12','DD-MON-RR')
GROUP BY account_id
);

------解决方案--------------------
SQL code

Plan hash value: 3708590803
 
------------------------------------------------------------
| Id  | Operation            | Name                    | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------
|   0 | SELECT STATEMENT     |                         |     1 |    13 |     3  (34)| 00:00:01 |
|   1 |  SORT AGGREGATE      |                         |     1 |    13 |            |          |
|   2 |   VIEW               |                         |     1 |    13 |     3  (34)| 00:00:01 |
|   3 |    HASH GROUP BY     |                         |     1 |    17 |     3  (34)| 00:00:01 |
|*  4 |     TABLE ACCESS FULL|         tab             |     1 |    17 |     2   (0)| 00:00:01 |
------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   4 - filter(TRUNC(INTERNAL_FUNCTION("CREATED_AT"))=TO_DATE('17-FEB-12','DD-MON-RR'))

Unable to gather statistics please unsure user has correct access.
The statistic feature requires that the user is granted select on v_$sesstat, v_$statname and v_$session.

------解决方案--------------------