LAG

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（默认值：1）
• default：当不存在前一行时返回的值（默认值：NULL）

说明：

• 负的偏移值与 LEAD 函数的行为类似
• 如果偏移超出分区边界，则返回 NULL

示例

-- Create sample data
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);

获取每个学生上一次测试的分数：

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

获取前 2 次测试的分数：

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