Select which has max date or latest date Here are two tables.

SCHOOL_STAFF

SCHOOL_CODE + STAFF_TYPE_NAME + LAST_UPDATE_DATE_TIME + PERSON_ID
=================================================================
ABE           Principal         24-JAN-13               111222
ABE           Principal         09-FEB-12               222111


PERSONS

PERSON_ID + NAME
=================
111222      ABC
222111      XYZ


Here is my oracle query.

SELECT MAX(LAST_UPDATE_DATE_TIME) AS LAST_UPDATE, SCHOOL_CODE, PERSON_ID
FROM SCHOOL_STAFF
WHERE STAFF_TYPE_NAME='Principal'
GROUP BY SCHOOL_CODE, PERSON_ID
ORDER BY SCHOOL_CODE;


which gives this results

LAST_UPDATE SCHOOL_CODE PERSON_ID
===========+===========+=========
24-JAN-13   ABE         111222
09-FEB-12   ABE         222111


I want to select the first one for the school which has latest date.

Thanks.
Your current query is not giving the desired result because you are using a GROUP BY clause on the PERSON_ID column which has a unique value for both entries.  As a result you will return both rows. 

There are a few ways that you can solve this.  You can use a subquery to apply the aggregate function to return the max(LAST_UPDATE_DATE_TIME) for each SCHOOL_CODE:

select s1.LAST_UPDATE_DATE_TIME,
  s1.SCHOOL_CODE,
  s1.PERSON_ID
from SCHOOL_STAFF s1
inner join
(
  select max(LAST_UPDATE_DATE_TIME) LAST_UPDATE_DATE_TIME,
    SCHOOL_CODE
  from SCHOOL_STAFF
  group by SCHOOL_CODE
) s2
  on s1.SCHOOL_CODE = s2.SCHOOL_CODE
  and s1.LAST_UPDATE_DATE_TIME = s2.LAST_UPDATE_DATE_TIME;


See SQL Fiddle with Demo

Or you can use use a windowing function to return the rows of data for each school with the most recent LAST_UPDATE_DATE_TIME:

select SCHOOL_CODE, PERSON_ID, LAST_UPDATE_DATE_TIME
from
(
  select SCHOOL_CODE, PERSON_ID, LAST_UPDATE_DATE_TIME,
    row_number() over(partition by SCHOOL_CODE 
                        order by LAST_UPDATE_DATE_TIME desc) seq
  from SCHOOL_STAFF
  where STAFF_TYPE_NAME='Principal'
) d
where seq = 1;


See SQL Fiddle with Demo

This query implements row_number() which assigns a unique number to each row in the partition of SCHOOL_CODE and placed in a descending order based on the LAST_UPDATE_DATE_TIME. 

As a side note, the JOIN with aggregate function is not exactly the same as the row_number() version.  If you have two rows with the same event time the JOIN will return both rows, while the row_number() will only return one.  If you want to return both with a windowing function, then consider using the rank() windowing function instead as it will return ties:

select SCHOOL_CODE, PERSON_ID, LAST_UPDATE_DATE_TIME
from
(
  select SCHOOL_CODE, PERSON_ID, LAST_UPDATE_DATE_TIME,
    rank() over(partition by SCHOOL_CODE 
                        order by LAST_UPDATE_DATE_TIME desc) seq
  from SCHOOL_STAFF
  where STAFF_TYPE_NAME='Principal'
) d
where seq = 1;


See Demo