How Can I Calculate the Difference Between Consecutive Rows in SQL Using Window Functions?

Efficiently Calculating Differences Between Adjacent SQL Rows with Window Functions

Data analysis frequently involves determining the difference between successive rows within a dataset. SQL's window functions offer an elegant solution, particularly the LAG function.

Leveraging the LAG Function

The LAG function retrieves a value from a preceding row within a defined window. The OVER clause specifies the window's ordering (e.g., ORDER BY Id). LAG then returns the designated column's value from the row immediately before the current row in the specified order.

The following SQL query illustrates calculating the difference between consecutive "value" entries in a table named "table":

SELECT value - LAG(value) OVER (ORDER BY Id) AS difference FROM table;

This generates a new table showing the difference between each row's "value" and its predecessor's "value".

Addressing Potential Gaps in ID Sequences

Using an ID column for ordering requires awareness of potential gaps in the ID sequence. Directly subtracting the previous ID (Id-1) might yield inaccurate difference calculations if IDs are not consecutive.

Illustrative Output

Given a "table" with this data:

<code>Id | value
----- | -----
1 | 10
2 | 15
3 | 20
4 | 25</code>

The above SQL query produces:

<code>difference
----------
NULL
5
5
5</code>

Note the NULL for the first row, as there's no preceding row to compare against.

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