In SQL, The LEAD function is a window function that allows you to access the value of a subsequent row within the current row’s result set based on a specified ordering.
I have tried to explain it in the video below.
The syntax of the LEAD function is as follows:
LEAD(expression, offset, default) OVER (
[PARTITION BY partition_expression, ... ]
ORDER BY sort_expression [ASC | DESC], ...
)
Example Query:
In the given SQL query, the LEAD function is being used to retrieve the value of the Page Type column from the next row within the same Session Id partition, based on the ascending ordering of the PageOrder column
SELECT [Date] ,
[ClientID],
[SessionID],
[PageOrder],
[PageType],
lead([PageType],1) over(partition by sessionid order by pageorder asc) as nextPage
FROM [Practice ].[dbo].[Pages]expression: the column or expression from which to retrieve the next value.
offset: the number of rows to skip to access the next value. In the example query, 1 is used to retrieve the next row’s PageType value.
default: the value to return if the next row is not found. In the example query, the default value is not specified, so it will return NULL if the next row is not found.
partition_expression: the column or expression by which to partition the result set. In the example query, SessionID is used to partition the result set.
sort_expression: the column or expression by which to order the result set. In the example query, PageOrder is used to order the result set in ascending order.
Therefore, the nextPage column in the given SQL query will contain the value of the PageType column from the next row within the same SessionID partition, ordered by the PageOrder column in ascending order.