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 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)
  ORDER BY last_name;

DEPARTMENT_ID LAST_NAME                     SALARY LOWEST_SAL
------------- ------------------------- ---------- -------------------------
           90 De Haan                        17000 Kochhar
           90 King                           24000 Kochhar
           90 Kochhar                        17000 Kochhar

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 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)
  ORDER BY last_name;

DEPARTMENT_ID LAST_NAME                     SALARY FV
------------- ------------------------- ---------- -------------------------
           90 De Haan                        17000 De Haan
           90 King                           24000 De Haan
           90 Kochhar                        17000 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)
  ORDER BY last_name;

DEPARTMENT_ID LAST_NAME           SALARY HIRE_DATE FV
------------- --------------- ---------- --------- -------------------------
           90 De Haan              17000 13-JAN-01 De Haan
           90 King                 24000 17-JUN-03 De Haan
           90 Kochhar              17000 21-SEP-05 De Haan

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

SELECT department_id, last_name, salary,
       FIRST_VALUE(last_name)
         OVER (ORDER BY salary ASC RANGE 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 De Haan                        17000 De Haan
           90 Kochhar                        17000 De Haan
           90 King                           24000 De Haan