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LAG

Introduced: v1.1.50

LAG allows you to access the value of a column from a preceding row within the same result set. It is typically used to retrieve the value of a column in the previous row, based on a specified ordering.

See also: LEAD

Syntax​

LAG(expression [, offset [, default]]) OVER (PARTITION BY partition_expression ORDER BY sort_expression)

  • offset: Specifies the number of rows ahead (LEAD) or behind (LAG) the current row within the partition to retrieve the value from. Defaults to 1.

  • default: Specifies a value to be returned if the LEAD or LAG function encounters a situation where there is no value available due to the offset exceeding the partition's boundaries. Defaults to NULL.

Examples​

CREATE TABLE sales (
  sale_id INT,
  product_name VARCHAR(50),
  sale_amount DECIMAL(10, 2)
);

INSERT INTO sales (sale_id, product_name, sale_amount)
VALUES (1, 'Product A', 1000.00),
       (2, 'Product A', 1500.00),
       (3, 'Product A', 2000.00),
       (4, 'Product B', 500.00),
       (5, 'Product B', 800.00),
       (6, 'Product B', 1200.00);

SELECT product_name, sale_amount, LAG(sale_amount) OVER (PARTITION BY product_name ORDER BY sale_id) AS previous_sale_amount
FROM sales;

product_name | sale_amount | previous_sale_amount
-----------------------------------------------
Product A    | 1000.00     | NULL
Product A    | 1500.00     | 1000.00
Product A    | 2000.00     | 1500.00
Product B    | 500.00      | NULL
Product B    | 800.00      | 500.00
Product B    | 1200.00     | 800.00