FIRST_VALUE is an non-deterministic analytic function.
It returns the first value in an ordered set of values.
If the first value in the set is null, then the function returns NULL unless you specify IGNORE NULLS.
This setting is useful for data densification. If you specify IGNORE NULLS, then FIRST_VALUE returns the fist non-null value in the set, or NULL if all values are null.
The following example selects, for each employee in Department 90, the name of the employee with the lowest salary.
SELECT department_id, last_name, salary, FIRST_VALUE(last_name)
OVER (ORDER BY salary ASC ROWS UNBOUNDED PRECEDING) AS lowest_sal
FROM (SELECT * FROM employees WHERE department_id = 90
ORDER BY employee_id);
DEPARTMENT_ID LAST_NAME SALARY LOWEST_SAL
------------- ------------- ---------- -------------------------
90 Kochhar 17000 Kochhar
90 De Haan 17000 Kochhar
90 King 24000 Kochhar
The example illustrates the non-deterministic nature of the FIRST_VALUE function. Kochhar and DeHaan have the same salary, so are in adjacent rows. Kochhar appears first because the rows returned by the subquery are ordered by employee_id. However, if the rows returned by the subquery are ordered by employee_id in descending order, as in the next example, then the function returns a different value:
SELECT department_id, last_name, salary, FIRST_VALUE(last_name)
OVER (ORDER BY salary ASC ROWS UNBOUNDED PRECEDING) as fv
FROM (SELECT * FROM employees WHERE department_id = 90
ORDER by employee_id DESC);
DEPARTMENT_ID LAST_NAME SALARY FV
------------- ------------- ---------- -------------------------
90 De Haan 17000 De Haan
90 Kochhar 17000 De Haan
90 King 24000 De Haan
The following example shows how to make the FIRST_VALUE function deterministic by ordering on a unique key.
SELECT department_id, last_name, salary, hire_date,
FIRST_VALUE(last_name) OVER
(ORDER BY salary ASC, hire_date ROWS UNBOUNDED PRECEDING) AS fv
FROM (SELECT * FROM employees
WHERE department_id = 90 ORDER BY employee_id DESC);
DEPARTMENT_ID LAST_NAME SALARY HIRE_DATE FV
------------- ------------- ---------- --------- ---------------
90 Kochhar 17000 21-SEP-89 Kochhar
90 De Haan 17000 13-JAN-93 Kochhar
90 King 24000 17-JUN-87 Kochhar