How to Calculate the Value Difference Between Consecutive Rows in a Database Table Using SQL?

Calculating the Difference Between Consecutive Rows in a Database Table Using SQL

Let's say you have a database table with a non-sequential integer as the row identifier (rowInt) and a numeric value (Value). The challenge is to efficiently calculate the difference between the Value of consecutive rows using SQL.

A simple query like this will retrieve the data:

SELECT * FROM myTable ORDER BY rowInt;

However, we need to compute and display the difference. The desired output format is:

<code>rowInt   Value   Diff
2        23      22    --45-23
3        45     -35    --10-45
9        10     -45    --0-45
17       0       0     --0 (or NULL, depending on desired behavior)</code>

Here's how to achieve this using a self-join with a subquery:

SELECT
    c.rowInt,
    c.Value,
    ISNULL(n.Value, 0) - c.Value AS Diff
FROM
    myTable AS c
LEFT JOIN
    myTable AS n ON n.rowInt = (SELECT MIN(rowInt) FROM myTable WHERE rowInt > c.rowInt)
ORDER BY c.rowInt;

This query performs a LEFT JOIN of the table (myTable) onto itself, aliased as c (current) and n (next). The subquery within the ON clause finds the minimum rowInt greater than the current row's rowInt, effectively identifying the next row. ISNULL(n.Value, 0) handles the case where there's no next row (for the last row), substituting 0 for the missing n.Value. The difference is then calculated and aliased as Diff. The final ORDER BY clause ensures the results are presented in the correct order. Alternatively, you could use COALESCE instead of ISNULL. The choice depends on your specific database system.

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