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.

For the optional offset argument, specify an integer that is greater than zero. 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 purchasing clerk in the employees table, the salary of the employee hired just before:

SELECT hire_date, last_name, salary,
 LAG(salary, 1, 0) OVER (ORDER BY hire_date) AS prev_sal
 FROM employees
 WHERE job_id = 'PU_CLERK'
 ORDER BY hire_date;
 
HIRE_DATE LAST_NAME SALARY PREV_SAL
--------- ------------------------- ---------- ----------
18-MAY-03 Khoo 3100 0
24-JUL-05 Tobias 2800 3100
24-DEC-05 Baida 2900 2800
15-NOV-06 Himuro 2600 2900
10-AUG-07 Colmenares 2500 2600