Hive LAG Function: Usage & Examples

In Hive, the LAG function is used to retrieve the value of a specific column from the previous row. The function takes three parameters: the column name to query, the offset to search for (i.e. the number of rows to retrieve, default is 1), and an optional default value to return if the previous row is not found. Here is the basic syntax of the LAG function:

LAG(column_name, offset, default_value) OVER (PARTITION BY partition_col ORDER BY order_col)

Among them, column_name represents the column to be queried, offset represents the offset of the number of rows to be obtained, and default_value represents an optional default value. The PARTITION BY clause is used to specify the partition column, and the ORDER BY clause is used to specify the sorting column.

Here is an example using the LAG function:

SELECT 
  id,
  revenue,
  LAG(revenue, 1, 0) OVER (ORDER BY id) AS previous_revenue
FROM 
  sales_data;

In this example, we queried a sales_data table to retrieve the values of the revenue column for each row, and used the LAG function to obtain the revenue value of the previous row. If the previous row does not exist (i.e. the current row is the first row), it returns the default value of 0.