LAST_VALUE
引入或更新于：v1.2.697
返回窗口框架中的最后一个值。
另请参阅：
语法
LAST_VALUE(expression) [ { RESPECT | IGNORE } NULLS ]
OVER (
[ PARTITION BY partition_expression ]
ORDER BY sort_expression [ ASC | DESC ]
[ window_frame ]
)
参数：
expression：必需。要返回最后一个值的列或表达式。PARTITION BY：可选。将行划分为分区（Partition）。ORDER BY：必需。确定窗口内的排序方式。window_frame：可选。定义窗口框架。默认值为RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW。
说明：
- 返回有序窗口框架中的最后一个值。
- 支持使用
IGNORE NULLS跳过空值，使用RESPECT NULLS保持默认行为。 - 当需要获取分区（Partition）的真正最后一行时，请使用在当前行之后结束的框架（例如，
ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING）。 - 可用于查找每个组中的最新值，或在向前看的窗口中查找最近的值。
示例
-- 示例订单数据
CREATE OR REPLACE TABLE orders_window_demo (
customer VARCHAR,
order_id INT,
order_time TIMESTAMP,
amount INT,
sales_rep VARCHAR
);
INSERT INTO orders_window_demo VALUES
('Alice', 1001, to_timestamp('2024-05-01 09:00:00'), 120, 'Erin'),
('Alice', 1002, to_timestamp('2024-05-01 11:00:00'), 135, NULL),
('Alice', 1003, to_timestamp('2024-05-02 14:30:00'), 125, 'Glen'),
('Bob', 1004, to_timestamp('2024-05-01 08:30:00'), 90, NULL),
('Bob', 1005, to_timestamp('2024-05-01 20:15:00'), 105, 'Kai'),
('Bob', 1006, to_timestamp('2024-05-03 10:00:00'), 95, NULL),
('Carol', 1007, to_timestamp('2024-05-04 09:45:00'), 80, 'Lily');
示例 1：每个客户分区中的最新订单
SELECT customer,
order_id,
order_time,
LAST_VALUE(order_id) OVER (
PARTITION BY customer
ORDER BY order_time
ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
) AS last_order_for_customer
FROM orders_window_demo
ORDER BY customer, order_time;
结果：
customer | order_id | order_time | last_order_for_customer
---------+----------+----------------------+-------------------------
Alice | 1001 | 2024-05-01 09:00:00 | 1003
Alice | 1002 | 2024-05-01 11:00:00 | 1003
Alice | 1003 | 2024-05-02 14:30:00 | 1003
Bob | 1004 | 2024-05-01 08:30:00 | 1006
Bob | 1005 | 2024-05-01 20:15:00 | 1006
Bob | 1006 | 2024-05-03 10:00:00 | 1006
Carol | 1007 | 2024-05-04 09:45:00 | 1007
示例 2：在每个客户内向前查看 12 小时
SELECT customer,
order_id,
order_time,
amount,
LAST_VALUE(amount) OVER (
PARTITION BY customer
ORDER BY order_time
RANGE BETWEEN CURRENT ROW AND INTERVAL 12 HOUR FOLLOWING
) AS last_amount_next_12h
FROM orders_window_demo
ORDER BY customer, order_time;
结果：
customer | order_id | order_time | amount | last_amount_next_12h
---------+----------+----------------------+--------+----------------------
Alice | 1001 | 2024-05-01 09:00:00 | 120 | 135
Alice | 1002 | 2024-05-01 11:00:00 | 135 | 135
Alice | 1003 | 2024-05-02 14:30:00 | 125 | 125
Bob | 1004 | 2024-05-01 08:30:00 | 90 | 105
Bob | 1005 | 2024-05-01 20:15:00 | 105 | 105
Bob | 1006 | 2024-05-03 10:00:00 | 95 | 95
Carol | 1007 | 2024-05-04 09:45:00 | 80 | 80
示例 3：向前扫描最后一个销售代表时跳过空值
SELECT customer,
order_id,
sales_rep,
LAST_VALUE(sales_rep) RESPECT NULLS OVER (
PARTITION BY customer
ORDER BY order_time
ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
) AS last_rep_respect,
LAST_VALUE(sales_rep) IGNORE NULLS OVER (
PARTITION BY customer
ORDER BY order_time
ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
) AS last_rep_ignore
FROM orders_window_demo
ORDER BY customer, order_id;
结果：
customer | order_id | sales_rep | last_rep_respect | last_rep_ignore
---------+----------+-----------+------------------+-----------------
Alice | 1001 | Erin | Glen | Glen
Alice | 1002 | NULL | Glen | Glen
Alice | 1003 | Glen | Glen | Glen
Bob | 1004 | NULL | NULL | Kai
Bob | 1005 | Kai | NULL | Kai
Bob | 1006 | NULL | NULL | Kai
Carol | 1007 | Lily | Lily | Lily
