What is the usage of the Oracle lag function?

2 years ago
Ava Mitchell
2 minutes

The LAG function is a window function in Oracle SQL that is used to access data from a previous row in the same result set. It’s particularly useful for comparing values in the current row with values in a preceding row, such as calculating differences or identifying trends.

Syntax

The basic syntax of the LAG function is as follows:

LAG(expression, offset, default) OVER (PARTITION BY partition_expression ORDER BY sort_expression)
  • expression: The column or expression for which you want to retrieve the value from a previous row.
  • offset: The number of rows back from the current row from which to retrieve the value. If omitted, the default value is 1.
  • default: The value to return if the offset goes beyond the first row of the partition. If omitted, the function returns NULL.
  • PARTITION BY: An optional clause that divides the result set into partitions to which the function is applied.
  • ORDER BY: Specifies the order of rows within each partition. The LAG function operates according to this order.

Example

Suppose you have a table named sales with columns sale_date and amount, and you want to calculate the difference in sales amount between each day and the previous day. You can use the LAG function as follows:

SELECT
 sale_date,
 amount,
 LAG(amount, 1, 0) OVER (ORDER BY sale_date) AS previous_amount,
 amount - LAG(amount, 1, 0) OVER (ORDER BY sale_date) AS amount_difference
FROM
 sales;

In this example:

  • LAG(amount, 1, 0) OVER (ORDER BY sale_date) retrieves the sales amount from the previous day, ordered by sale_date. If there’s no previous day (i.e., for the first row), it returns 0.
  • amount - LAG(amount, 1, 0) OVER (ORDER BY sale_date) calculates the difference between the current day’s sales amount and the previous day’s sales amount.

Use Cases

The LAG function is useful in a variety of scenarios, including:

  • Calculating running totals or moving averages.
  • Identifying trends or patterns in time-series data.
  • Comparing values across rows in a report.
  • Implementing custom ranking or numbering schemes.

Conclusion

The LAG function is a powerful tool for analyzing data in Oracle SQL. By understanding its syntax and use cases, you can perform complex calculations and gain valuable insights from your data.

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