LAG

Syntax

Purpose

LAG is an analytic function. It provides access to more than one row of a table at the same time without a self join. Given a series of rows returned from a query and a position of the cursor, LAG provides access to a row at a given physical offset prior to that position.

If you do not specify offset, then its default is 1. The optional default value is returned if the offset goes beyond the scope of the window. If you do not specify default, then its default is null.

{RESPECT | IGNORE} NULLS determines whether null values of value_expr are included in or eliminated from the calculation. The default is RESPECT NULLS.

You cannot nest analytic functions by using LAG or any other analytic function for value_expr. However, you can use other built-in function expressions for value_expr.

Examples

The following example provides, for each salesperson in the employees table, the salary of the employee hired just before:

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'
   ORDER BY last_name, hire_date, salary, prev_sal;
   
LAST_NAME                 HIRE_DATE     SALARY   PREV_SAL
------------------------- --------- ---------- ----------
Baida                     24-DEC-05       2900       2800
Colmenares                10-AUG-07       2500       2600
Himuro                    15-NOV-06       2600       2900
Khoo                      18-MAY-03       3100          0
Tobias                    24-JUL-05       2800       3100