LAG
引入或更新于:v1.2.45
返回结果集中前一行的值。
另请参阅:LEAD
语法
LAG(
expression
[, offset ]
[, default ]
)
OVER (
[ PARTITION BY partition_expression ]
ORDER BY sort_expression
)
参数:
expression
:要计算的列或表达式offset
:当前行之前的行数(默认:1)default
:当不存在前一行时返回的值(默认:NULL)
说明:
- 负偏移量值的行为类似于 LEAD 函数
- 如果偏移量超出分区边界,则返回 NULL
示例
-- 创建示例数据
CREATE TABLE scores (
student VARCHAR(20),
test_date DATE,
score INT
);
INSERT INTO scores VALUES
('Alice', '2024-01-01', 85),
('Alice', '2024-02-01', 90),
('Alice', '2024-03-01', 88),
('Bob', '2024-01-01', 78),
('Bob', '2024-02-01', 82),
('Bob', '2024-03-01', 85);
获取每个学生的上一次考试成绩:
SELECT student, test_date, score,
LAG(score) OVER (PARTITION BY student ORDER BY test_date) AS previous_score
FROM scores
ORDER BY student, test_date;
结果:
student | test_date | score | previous_score
--------+------------+-------+---------------
Alice | 2024-01-01 | 85 | NULL
Alice | 2024-02-01 | 90 | 85
Alice | 2024-03-01 | 88 | 90
Bob | 2024-01-01 | 78 | NULL
Bob | 2024-02-01 | 82 | 78
Bob | 2024-03-01 | 85 | 82
获取两次考试前的成绩:
SELECT student, test_date, score,
LAG(score, 2, 0) OVER (PARTITION BY student ORDER BY test_date) AS score_2_tests_ago
FROM scores
ORDER BY student, test_date;
结果:
student | test_date | score | score_2_tests_ago
--------+------------+-------+------------------
Alice | 2024-01-01 | 85 | 0
Alice | 2024-02-01 | 90 | 0
Alice | 2024-03-01 | 88 | 85
Bob | 2024-01-01 | 78 | 0
Bob | 2024-02-01 | 82 | 0
Bob | 2024-03-01 | 85 | 78