Oracle/PLSQL: 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.

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.

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

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

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:

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:

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