Oracle PL/SQL: Lag Function

In Oracle/PLSQL, the lag function is an analytic function that lets you query more than one row in a table at a time without having to join the table to itself. It returns values from a previous row in the table. To return a value from the next row, try using the lead function.

Syntax

The syntax for the lag function is:

lag ( expression [, offset [, default] ] )
over ( [ query_partition_clause ] order_by_clause )

expression is an expression that can contain other built-in functions, but can not contain any analytic functions.

offset is optional. It is the physical offset from the current row in the table. If this parameter is omitted, the default is 1.

default is optional. It is the value that is returned if the offset goes out of the bounds of the table. If this parameter is omitted, the default is null.

Applies To

• Oracle 11g, Oracle 10g, Oracle 9i, Oracle 8i

For Example

Let's take a look at an example. If we had an orders table that contained the following data:

ORDER_DATE	PRODUCT_ID	QTY
25/09/2007	1000	20
26/09/2007	2000	15
27/09/2007	1000	8
28/09/2007	2000	12
29/09/2007	2000	2
30/09/2007	1000	4

And we ran the following SQL statement:

select product_id, order_date,
lag (order_date,1) over (ORDER BY order_date) AS prev_order_date
from orders;

It would return the following result:

PRODUCT_ID	ORDER_DATE	PREV_ORDER_DATE
1000	25/09/2007	<NULL>
2000	26/09/2007	25/09/2007
1000	27/09/2007	26/09/2007
2000	28/09/2007	27/09/2007
2000	29/09/2007	28/09/2007
1000	30/09/2007	29/09/2007

Since we used an offset of 1, the query returns the previous order_date.

If we had used an offset of 2 instead, it would have returned the order_date from 2 orders before. If we had used an offset of 3, it would have returned the order_date from 3 orders before....and so on.

If we wanted only the orders for a given product_id, we could run the following SQL statement:

select product_id, order_date,
lag (order_date,1) over (ORDER BY order_date) AS prev_order_date
from orders
where product_id = 2000;

It would return the following result:

PRODUCT_ID	ORDER_DATE	PREV_ORDER_DATE
2000	26/09/2007	<NULL>
2000	28/09/2007	26/09/2007
2000	29/09/2007	28/09/2007

In this example, it returned the previous order_date for product_id = 2000 and ignored all other orders.