mysql group 陷阱

mysql> SELECT
->             sn,Clientsn,createdtime
->         FROM
->             ClientCashDetail
->         WHERE
->             createdTime < '2015-12-25 00:00:00'
->         GROUP BY clientSn
->         ORDER BY createdtime DESC;
+-------+----------+---------------------+
| sn    | Clientsn | createdtime         |
+-------+----------+---------------------+
| 20146 |    12904 | 2015-12-28 18:51:59 |
| 20143 |      501 | 2015-12-28 17:36:21 |
| 20141 |     1177 | 2015-12-28 17:12:55 |
| 20135 |       57 | 2015-12-28 16:47:43 |
| 20124 |     1407 | 2015-12-28 16:19:15 |
| 20108 |    12808 | 2015-12-28 16:05:52 |
| 20107 |      592 | 2015-12-28 16:05:33 |
| 20102 |    12738 | 2015-12-28 16:03:21 |
| 20094 |     1570 | 2015-12-28 15:59:33 |
| 20091 |     2102 | 2015-12-28 15:57:18 |
| 20086 |     1659 | 2015-12-28 15:50:31 |
| 20084 |    13006 | 2015-12-28 15:07:03 |
| 20083 |      512 | 2015-12-28 15:04:51 |

mysql> select Clientsn ,count(*)  from ClientCashDetail group by Clientsn order by count(*) ;
|       12 |      100 |
|      392 |      100 |
|      253 |      105 |
|      605 |      105 |
|       55 |      107 |
|       22 |      109 |
|      592 |      110 |
|       65 |      110 |
|       31 |      112 |
|       71 |      113 |
|       57 |      118 |
|      127 |      123 |
|        1 |      127 |
|       45 |      140 |
|      351 |      156 |
|       15 |      160 |
|       13 |      187 |
|     1123 |      198 |
+----------+----------+
1075 rows in set (0.00 sec)

mysql> select sn,Clientsn,createdtime from ClientCashDetail where  createdTime >= '2015-12-24 00:00:00' and Clientsn=1123;
+-------+----------+---------------------+
| sn    | Clientsn | createdtime         |
+-------+----------+---------------------+
| 19951 |     1123 | 2015-12-24 14:36:20 |
| 19995 |     1123 | 2015-12-24 15:11:59 |
| 19996 |     1123 | 2015-12-24 15:19:45 |
| 20118 |     1123 | 2015-12-28 16:09:24 |
+-------+----------+---------------------+
4 rows in set (0.00 sec)

mysql>  select sn,Clientsn,createdtime from ClientCashDetail where  createdTime >= '2015-12-24 00:00:00' and Clientsn=1123 group by Clientsn;
+-------+----------+---------------------+
| sn    | Clientsn | createdtime         |
+-------+----------+---------------------+
| 19951 |     1123 | 2015-12-24 14:36:20 |
+-------+----------+---------------------+
1 row in set (0.00 sec)

mysql>  select sn,Clientsn,createdtime from ClientCashDetail where  createdTime >= '2015-12-24 00:00:00' and Clientsn=1123 group by Clientsn order by createdtime desc;
+-------+----------+---------------------+
| sn    | Clientsn | createdtime         |
+-------+----------+---------------------+
| 19951 |     1123 | 2015-12-24 14:36:20 |
+-------+----------+---------------------+
1 row in set (0.00 sec)

-------------------------------------------------------------------------------------------------------------------------------------------------------------------
mysql>  select sn,Clientsn,createdtime from ClientCashDetail where  createdTime >= '2015-12-24 00:00:00' and Clientsn=1123 ;
+-------+----------+---------------------+
| sn    | Clientsn | createdtime         |
+-------+----------+---------------------+
| 19951 |     1123 | 2015-12-24 14:36:20 |
| 19995 |     1123 | 2015-12-24 15:11:59 |
| 19996 |     1123 | 2015-12-24 15:19:45 |
| 20118 |     1123 | 2015-12-28 16:09:24 |
+-------+----------+---------------------+
4 rows in set (0.00 sec)

mysql>  select max(sn),Clientsn,max(createdtime) from ClientCashDetail where  createdTime >= '2015-12-24 00:00:00' and Clientsn=1123 group by Clientsn ;
+---------+----------+---------------------+
| max(sn) | Clientsn | max(createdtime)    |
+---------+----------+---------------------+
|   20118 |     1123 | 2015-12-28 16:09:24 |
+---------+----------+---------------------+
1 row in set (0.00 sec)

|    12825 | 2015-12-24 12:05:53 |
|    12861 | 2015-12-24 11:49:03 |
|    12899 | 2015-12-24 09:12:15 |
|    12904 | 2015-12-28 19:00:04 |
|    12914 | 2015-12-25 13:53:19 |
|    12941 | 2015-12-26 01:57:16 |
|    13004 | 2015-12-28 10:08:04 |
|    13006 | 2015-12-28 16:06:18 |
+----------+---------------------+
1075 rows in set (0.00 sec)

mysql> select ClientSn, max(createdtime) from ClientCashDetail group by ClientSn;

select  SUM(accountBalance)  from ( SELECT
a.*
FROM
ClientCashDetail a
INNER JOIN
(SELECT
MAX(createdtime) AS mtime
FROM
ClientCashDetail
where createdTime < '2015-12-25 00:00:00'
GROUP BY ClientSn) b ON a.createdtime = b.mtime) c