FIRST_VALUE

Syntax

Purpose

FIRST_VALUE is an 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.

{RESPECT | IGNORE} NULLS determines whether null values of expr are included in or eliminated from the calculation. The default is RESPECT NULLS. If you specify IGNORE NULLS, then FIRST_VALUE returns the first non-null value in the set, or NULL if all values are null. Refer to "Using Partitioned Outer Joins: Examples" for an example of data densification.

You cannot nest analytic functions by using FIRST_VALUE or any other analytic function for expr. However, you can use other built-in function expressions for expr. Refer to "About SQL Expressions" for information on valid forms of expr.

Examples

The following example selects, for each employee in Department 90, the name of the employee with the lowest salary.

SELECT employee_id, last_name, salary, hire_date,
 FIRST_VALUE(last_name)
 OVER (ORDER BY salary ASC ROWS UNBOUNDED PRECEDING) AS fv
 FROM (SELECT * FROM employees
 WHERE department_id = 90
 ORDER BY hire_date);
EMPLOYEE_ID LAST_NAME SALARY HIRE_DATE FV
----------- ------------------------- ---------- --------- -------
 102 De Haan 17000 13-JAN-01 De Haan
 101 Kochhar 17000 21-SEP-05 De Haan
 100 King 24000 17-JUN-03 De Haan

The example illustrates the nondeterministic 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 hire_date. However, if the rows returned by the subquery are ordered by hire_date in descending order, as in the next example, then the function returns a different value:

SELECT employee_id, last_name, salary, hire_date,
 FIRST_VALUE(last_name)
 OVER (ORDER BY salary ASC ROWS UNBOUNDED PRECEDING) AS fv
 FROM (SELECT * FROM employees
 WHERE department_id = 90
 ORDER by hire_date DESC);
EMPLOYEE_ID LAST_NAME SALARY HIRE_DATE FV
----------- ------------------------- ---------- --------- -------
 101 Kochhar 17000 21-SEP-05 Kochhar
 102 De Haan 17000 13-JAN-01 Kochhar
 100 King 24000 17-JUN-03 Kochhar

The following two examples show how to make the FIRST_VALUE function deterministic by ordering on a unique key. By ordering within the function by both salary and the unique key employee_id, you can ensure the same result regardless of the ordering in the subquery.

SELECT employee_id, last_name, salary, hire_date,
 FIRST_VALUE(last_name)
 OVER (ORDER BY salary ASC, employee_id ROWS UNBOUNDED PRECEDING) AS fv
 FROM (SELECT * FROM employees
 WHERE department_id = 90
 ORDER BY hire_date);
 
EMPLOYEE_ID LAST_NAME SALARY HIRE_DATE FV
----------- ------------------------- ---------- --------- -------
 101 Kochhar 17000 21-SEP-05 Kochhar
 102 De Haan 17000 13-JAN-01 Kochhar
 100 King 24000 17-JUN-03 Kochhar
 
 
SELECT employee_id, last_name, salary, hire_date,
 FIRST_VALUE(last_name)
 OVER (ORDER BY salary ASC, employee_id ROWS UNBOUNDED PRECEDING) AS fv
 FROM (SELECT * FROM employees
 WHERE department_id = 90
 ORDER BY hire_date DESC);
 
EMPLOYEE_ID LAST_NAME SALARY HIRE_DATE FV
----------- ------------------------- ---------- --------- -------
 101 Kochhar 17000 21-SEP-05 Kochhar
 102 De Haan 17000 13-JAN-01 Kochhar
 100 King 24000 17-JUN-03 Kochhar

The following two examples show that the FIRST_VALUE function is deterministic when you use a logical offset (RANGE instead of ROWS). When duplicates are found for the ORDER BY expression, the FIRST_VALUE is the lowest value of expr:

SELECT employee_id, last_name, salary, hire_date,
 FIRST_VALUE(last_name)
 OVER (ORDER BY salary ASC RANGE UNBOUNDED PRECEDING) AS fv
 FROM (SELECT * FROM employees
 WHERE department_id = 90
 ORDER BY hire_date);
 
EMPLOYEE_ID LAST_NAME SALARY HIRE_DATE FV
----------- ------------------------- ---------- --------- -------
 102 De Haan 17000 13-JAN-01 De Haan
 101 Kochhar 17000 21-SEP-05 De Haan
 100 King 24000 17-JUN-03 De Haan
 
 
SELECT employee_id, last_name, salary, hire_date,
 FIRST_VALUE(last_name)
 OVER (ORDER BY salary ASC RANGE UNBOUNDED PRECEDING) AS fv
 FROM (SELECT * FROM employees
 WHERE department_id = 90
 ORDER BY hire_date DESC);
 
EMPLOYEE_ID LAST_NAME SALARY HIRE_DATE FV
----------- ------------------------- ---------- --------- -------
 102 De Haan 17000 13-JAN-01 De Haan
 101 Kochhar 17000 21-SEP-05 De Haan
 100 King 24000 17-JUN-03 De Haan