In this post, I am going to demonstrate one of new LAG() function which is introduced by SQL Server 2012.

Generally, we are using Self Join to perform row-level analysis by comparing Next and Previous rows.
SQL Server 2012 has a LAG() function is used for accessing data from the Previous Row in the same result set.

For example, we have a different transaction date in our table and require to find the total number of days difference between current and previous transaction date. We can use a LAG() function to achieve this.

Below is a full demonstration on this:

The Syntax:

LAG (scalar_expression [,offset] [,default])
    OVER ( [ partition_by_clause ] order_by_clause )

The default OFFSET value is 1 means considering next immediate row and we can also change this OFFSET value. e.g. set OFFSET value of 4 means it accesses immediate fourth row.

Create a sample table with data:

CREATE TABLE tbl_LEADDemo
(
	ID INTEGER IDENTITY(1,1)
	,TransactionDateTime DATETIME
)
GO

INSERT INTO tbl_LEADDemo (TransactionDateTime)
VALUES 
('2016-01-08 08:08:08')
,('2016-01-10 14:23:23')
,('2016-01-12 09:41:12')
,('2016-01-16 23:45:12')
,('2016-01-26 16:16:45')
,('2016-01-29 12:55:02')
,('2016-02-02 18:25:04')
,('2016-02-06 16:42:32')
GO

Calculate day difference using LAG():

;WITH cteLeadDemo AS
(
	SELECT 
		ID
		,TransactionDateTime AS NextDay
		,LAG(TransactionDateTime,1,0) OVER(ORDER BY TransactionDateTime) AS PreviousDay
	FROM tbl_LEADDemo
)
SELECT 
	ID
	,NextDay
	,PreviousDay
	,CASE 
		WHEN PreviousDay = '1900-01-01 00:00:00.000'
			THEN 0 
		ELSE DATEDIFF(DAY,PreviousDay,NextDay) 
	END AS TransactionDayInterval	
FROM cteLeadDemo
GO

The Result:

In this post, I am sharing the demonstration on the new LEAD() function which is introduced by SQL Server 2012.

Generally, we are using Self Join to perform row-level analysis by comparing Next and Previous rows.

SQL Server 2012 has a LEAD() function, is used for accessing data from the Next Row in the same result set.

For example, we have a different transaction date in our table and need to find the total number of days difference between current and next transaction date. We can use a LEAD() function to accomplish this.

Below is a full demonstration on this:

The Syntax:

LEAD (scalar_expression [,offset] [,default])
    OVER ( [ partition_by_clause ] order_by_clause )

The default OFFSET value is 1 means considering next immediate row and we can also change this OFFSET value. e.g. set OFFSET value of 4 means it accesses immediate fourth row.

Create a sample table with data:

CREATE TABLE tbl_LEADDemo
(
	ID INTEGER IDENTITY(1,1)
	,TransactionDateTime DATETIME
)
GO

INSERT INTO tbl_LEADDemo (TransactionDateTime)
VALUES 
('2016-01-08 08:08:08')
,('2016-01-10 14:23:23')
,('2016-01-12 09:41:12')
,('2016-01-16 23:45:12')
,('2016-01-26 16:16:45')
,('2016-01-29 12:55:02')
,('2016-02-02 18:25:04')
,('2016-02-06 16:42:32')
GO

Calculate day difference using LEAD():

;WITH cteLeadDemo AS
(
	SELECT 
		ID
		,TransactionDateTime AS PreviousDay
		,LEAD(TransactionDateTime,1,0) OVER(ORDER BY TransactionDateTime) AS NextDay
	FROM tbl_LEADDemo
)
SELECT 
	ID
	,PreviousDay
	,NextDay
	,CASE 
	    WHEN DATEDIFF(DAY,PreviousDay,NextDay) < 0 
		THEN 0 
	    ELSE DATEDIFF(DAY,PreviousDay,NextDay) 
	END AS TransactionDayInterval
FROM cteLeadDemo
GO

The Result: