SQL & PL/SQL :: How To Take Sign Off And Rest Field Value Of First Row For Same Staff

The below query is taken from the procedure

SELECT l_stafnum = stafnum,
category,
duty,
date,
signon,
signoff,
l_rest = rest,
valid,
FROM SESSION.valid_pf
WHERE invalid != 'I'

In the above query has set of data which each staff may have more than one duty.

For example

stafnum categoy duty date signon signoff rest valid
232 M a/c 2/3/12 5:00 8:00 12:00 I
232 M Sup 2/3/12 10:00 13:00 8:00 I
111 M Clen 2/3/12 9:00 12:00 12:00 I
232 M Sw 2/3/12 15:00 20:00 12:00 I
111 M Wp 2/3/12 10:00 14:00 06:00 I
121 M a/c 2/3/12 5:00 8:00 12:00 I

The select query will retrieve the value which mentioned this example.Now, First I need to take how many duty has been allocated for each stafnumber.

232 staff allocated to three duty (a/c,sup,sw) Then need to compare signoff value for first row addition of rest value for the same row i.e, i = 08:00+1200

Then need to compare value of signon for the same staff 232 for the second row which is 10:00 if (i>10:00) then valid;

how I can take same staff number and find it how many duty the staff has.And how to take sign off and rest field value of first row, for the same staff then compare those value with the same staff number second row signon value.