LAG
Returns the value of the input expression at the given offset before the current row within the requested window.
For more information on usage, please refer to Window Functions.
Syntax
LAG ( <expression> [, <offset> [, <default> ]] )
OVER ( [ PARTITION BY <partition_by> ] ORDER BY <order_by> [ { ASC | DESC } ] )
Parameters
| Parameter | Description | Supported input types |
|---|---|---|
<expression> | Any valid expression that will be returned based on the <offset>. | Any |
<offset> | The number of rows backward from the current row from which to obtain a value. A negative number will act as LEAD(). Must be a literal INTEGER. | INTEGER |
<default> | The expression to return when the offset goes out of the bounds of the window. Must be a literal of the same type as <expression>. The default is NULL. | Any |
<partition_by> | The expression used for the PARTITION BY clause. | Any |
<order_by> | An expression used for the ORDER BY clause. | Any |
Example
In the example below, the LAG function is being used to find the players in each level who ranked before and after a certain player. In some cases, if the player has no one ranked before or after them, the LAG function returns NULL.
SELECT
nickname,
level,
LAG(nickname, 1) OVER (PARTITION BY level ORDER BY nickname) AS player_before,
LAG(first_name, -1) OVER (PARTITION BY level ORDER BY nickname) AS player_after
FROM
players;
Returns:
| nickname | level | player_before | player_after |
|---|---|---|---|
| kennethpark | 9 | NULL | rileyjon |
| rileyjon | 9 | kennethpark | sabrina21 |
| sabrina21 | 9 | rileyjon | ymatthews |
| ymatthews | 9 | sabrina21 | NULL |