LAG (Lakehouse v1)

LAG 允许在同一结果集中访问前一行的列值。它通常用于按指定的排序检索上一行中某一列的值。

另请参阅：LEAD

Analyze 语法

func.lag(<expr>, <offset>).over(partition_by=[<columns>], order_by=[<columns>])

Analyze 示例

table.product_name, table.sale_amount, func.lag(table.sale_amount, 1).over(partition_by=table.product_name, order_by=table.sale_id).alias('previous_sale_amount')

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

SQL 语法

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

• offset：指定在分区内，相对于当前行向前（LEAD）或向后（LAG）的行数，从该行检索值。默认为 1。

请注意，将偏移量设为负数的效果与使用 LEAD 函数相同。

• default：指定当 LEAD 或 LAG 函数因偏移量超出分区边界而没有可用值时返回的值。默认为 NULL。

SQL 示例

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

-- The following statements return the same result.
SELECT product_name, sale_amount, LAG(sale_amount, -1) OVER (PARTITION BY product_name ORDER BY sale_id) AS next_sale_amount
FROM sales;

SELECT product_name, sale_amount, LEAD(sale_amount) OVER (PARTITION BY product_name ORDER BY sale_id) AS next_sale_amount
FROM sales;

product_name|sale_amount|next_sale_amount|
------------+-----------+----------------+
Product A   |    1000.00|         1500.00|
Product A   |    1500.00|         2000.00|
Product A   |    2000.00|                |
Product B   |     500.00|          800.00|
Product B   |     800.00|         1200.00|
Product B   |    1200.00|                |