Use and usage of Oracle lead and LGA two functions

Lead: Returns the value of the next record hire_date.
SELECT last_name, Hire_date,
Leads (hire_date, 1) over (order by hire_date) as "nexthired"
From Employees
WHERE department_id = 30;

Last_Name hire_date nexthired
-------------------------  ---------  ---------
raphaely 07-dec-94 18-may-95
Khoo 18-may-95 24-jul-97
Tobias 24-jul-97 24-dec-97
Baida 24-dec-97 15-nov-98
Himuro 15-nov-98 10-aug-99
Colmenares 10-aug-99

LAG: Returns the value of the previous record hire_date (if not, the value is 0).
SELECT last_name, hire_date, salary,
LAG (Salary, 1, 0) over (order by hire_date) as Prev_sal
From Employees
WHERE job_id = ' Pu_clerk ';


Last_Name hire_date SALARY Prev_sal
-------------------------  ---------  ----------  ----------
khoo                                            18-may-95              3100      &NB Sp