Understanding the Concept

When working with SQL databases, fetching the value of a column from the previous row is a common task. This can be achieved using specific SQL functions like LAG(), LEAD(), and window functions.

Using the LAG() Function

The LAG() function allows you to access data from the previous row in a dataset. For instance:

SELECT 
    column_name, 
    LAG(column_name) OVER (ORDER BY some_column) AS previous_value
FROM table_name;
            

This query retrieves the value of column_name from the previous row based on the order specified.

Benefits of Fetching Previous Row Values

• Track changes between rows.
• Compare values across consecutive rows.
• Generate cumulative results.

Practical Applications

This technique is widely used in analytics, financial calculations, and data transformations. By accessing the previous row value, you can calculate trends or perform data quality checks efficiently.