How Can I Access and Use Previous Row Values in SQL SELECT Statements?

Retrieving Previous Row Data within SQL SELECT Queries

This guide addresses the challenge of calculating differences between consecutive rows in a SQL table. A straightforward subtraction won't suffice due to the lack of inherent row order consideration in standard SQL. The solution involves leveraging the LAG function.

The LAG function retrieves data from preceding rows within a SELECT statement. Its syntax is:

<code class="language-sql">LAG(column_name, offset, default) OVER (ORDER BY order_column)</code>

Here's a breakdown:

• column_name: The column containing the data you need from the previous row.
• offset: Specifies how many rows to look back (1 for the immediately preceding row).
• default: The value returned if the offset exceeds the first row.
• order_column: The column defining the row order.

Example Implementation:

To calculate the difference between consecutive value entries, ordered by an Id column:

<code class="language-sql">SELECT value - LAG(value, 1, 0) OVER (ORDER BY Id) AS difference FROM your_table;</code>

This query subtracts the previous row's value (offset 1) from the current row's value. The Id column ensures correct ordering, preventing issues arising from non-sequential Id values. The 0 default handles the first row, avoiding errors. The result is a new difference column showcasing the calculated variations.

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