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(). The offset 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