Mail for error in tuples

Dear friend,
   I have a table named radius with structure

Table    = radius
+----------------------------------+----------------------------------+-------+

|              Field               |              Type                |
Length|
+----------------------------------+----------------------------------+-------+

| uname                            | char()
|   256 |
| logdate                          | date
|     4 |
| logtime                          | time
|     8 |
| duration                         | timespan
|    12 |
| status                           | char()
|    20 |
| nasadd                           | char()
|    20 |
| port                             | int4
|     4 |
| bytesin                          | int4
|     4 |
| bytesout                         | int4
|     4 |
| packin                           | int4
|     4 |
| packout                          | int4
|     4 |
| misc                             | int4
|     4 |
+----------------------------------+----------------------------------+-------+

I keep my radius log in this table.

Now in this logdate and logtime are the last logout date and time for
the particular user.

But sometimes the logdate and logtime and duration are wrong so inorder
to correct i have to do following can anyone help me how to do it.

I want to select the tuples of particular user and test the first
logdate and logtime with the next tuples logdate and logtime and the
duration in the second tuple should not be greater than the difference
of this two tuples if its greater than the difference then it needs to
be set to the difference of the two logdate and logtime. I have to test
it till the last tuple.
So can anyone suggest me query for this job.

Thanks in advance.

Regards,
Vicky