The LAG function is a powerful feature of SQLite that allows you to access data from a previous row in a result set. This can be useful when you need to compare values between rows or when you need to perform calculations based on previous values.
The syntax for the LAG function is as follows:
LAG(column_name, offset, default_value) OVER (PARTITION BY partition_clause ORDER BY order_clause)
column_name: the name of the column you want to access data from
offset: the number of rows to look back. For example, an offset of 1 means the previous row, 2 means two rows back, and so on.
default_value: the value to return if there is no previous row to retrieve data from.
PARTITION BY: optional clause that allows you to divide the result set into partitions based on one or more columns. The LAG function will operate on each partition separately.
ORDER BY: the column or columns to order the result set by. The LAG function will operate based on the ordering specified.
Here’s an example of how to use the LAG function:
Suppose you have a table named “sales” with the following columns: “date”, “product”, and “revenue”. You want to calculate the change in revenue between each day and the previous day. You can use the LAG function to retrieve the revenue from the previous row and then subtract it from the current row’s revenue:
SELECT date, product, revenue, revenue - LAG(revenue, 1, 0) OVER (PARTITION BY product ORDER BY date) AS revenue_change FROM sales;
In this example, we’re using an offset of 1 to retrieve the revenue from the previous row. We’re also partitioning the result set by the “product” column so that the LAG function operates on each product separately. Finally, we’re ordering the result set by the “date” column so that we’re always retrieving data from the previous day.
The result set will contain columns for “date”, “product”, “revenue”, and “revenue_change”, where “revenue_change” is the difference between the current row’s revenue and the previous row’s revenue. If there is no previous row to retrieve data from, the LAG function will return a default value of 0.
Overall, the LAG function is a useful tool for performing complex calculations on a result set in SQLite.