Retrieving Previous Row Data in SQL SELECT Statements
Many SQL queries require accessing data from the preceding row for calculations or comparisons. This article demonstrates how to accomplish this in Microsoft SQL Server 2008 using the LAG
function.
Leveraging the LAG Function
The LAG
function efficiently retrieves values from a specified column in a previous row. Its syntax is:
LAG(column_name, offset) OVER (ORDER BY order_column)
Here's a breakdown:
-
column_name
: The column containing the value you need. -
offset
: The number of rows to offset from the current row. A negative offset (e.g., -1) accesses the previous row. -
order_column
: The column defining the row order for the calculation.
Illustrative Example
To compute the difference between consecutive rows in a column named value
, use LAG
as follows:
SELECT value - LAG(value, 1) OVER (ORDER BY Id) AS ValueDifference FROM your_table;
This query subtracts the value
from the previous row (obtained via LAG(value, 1)
) from the current row's value
, producing the difference. ORDER BY Id
ensures the correct row order for the calculation.
Addressing Gaps in Sequence Numbers
It's crucial to remember that ID sequences might contain gaps. Directly referencing Id - 1
to find the previous row is unreliable in such cases. The LAG
function, however, handles these gaps seamlessly, providing a robust solution.
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