LAST_VALUE

描述

LAST_VALUE() 是一个窗口函数，用于返回窗口范围内的最后一个值。可以通过 IGNORE NULLS 选项来控制是否忽略空值。

语法

LAST_VALUE(<expr>[, <ignore_null>])

参数

参数	说明
expr	需要获取最后一个值的表达式
ignore_null	可选。参数 ignore_null 默认值为 false, 设置后会忽略空值

返回值

返回与输入表达式相同的数据类型。

举例

WITH example_data AS (
    SELECT 1 as id, 21 as myday, '04-21-11' as time_col, NULL as state
    UNION ALL
    SELECT 2, 21, '04-21-12', 2
    UNION ALL
    SELECT 3, 21, '04-21-13', 3
    UNION ALL
    SELECT 4, 22, '04-22-10-21', NULL
    UNION ALL
    SELECT 5, 22, '04-22-10-22', NULL
    UNION ALL
    SELECT 6, 22, '04-22-10-23', 5
    UNION ALL
    SELECT 7, 22, '04-22-10-24', NULL
    UNION ALL
    SELECT 8, 22, '04-22-10-25', 9
    UNION ALL
    SELECT 9, 23, '04-23-11', NULL
    UNION ALL
    SELECT 10, 23, '04-23-12', 10
    UNION ALL
    SELECT 11, 23, '04-23-13', NULL
    UNION ALL
    SELECT 12, 24, '02-24-10-21', NULL
)
SELECT 
    *,
    last_value(`state`, 1) OVER(
        PARTITION BY `myday` 
        ORDER BY `time_col` DESC 
        ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING
    ) as ignore_null,
    last_value(`state`, 0) OVER(
        PARTITION BY `myday` 
        ORDER BY `time_col` DESC 
        ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING
    ) as not_ignore_null,
    last_value(`state`) OVER(
        PARTITION BY `myday` 
        ORDER BY `time_col` DESC 
        ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING
    ) as ignore_null_default
FROM example_data 
ORDER BY `id`, `myday`, `time_col`;

+------+-------+-------------+-------+-------------+-----------------+---------------------+
| id   | myday | time_col    | state | ignore_null | not_ignore_null | ignore_null_default |
+------+-------+-------------+-------+-------------+-----------------+---------------------+
|    1 |    21 | 04-21-11    |  NULL |           2 |            NULL |                NULL |
|    2 |    21 | 04-21-12    |     2 |           2 |            NULL |                NULL |
|    3 |    21 | 04-21-13    |     3 |           2 |               2 |                   2 |
|    4 |    22 | 04-22-10-21 |  NULL |        NULL |            NULL |                NULL |
|    5 |    22 | 04-22-10-22 |  NULL |           5 |            NULL |                NULL |
|    6 |    22 | 04-22-10-23 |     5 |           5 |            NULL |                NULL |
|    7 |    22 | 04-22-10-24 |  NULL |           5 |               5 |                   5 |
|    8 |    22 | 04-22-10-25 |     9 |           9 |            NULL |                NULL |
|    9 |    23 | 04-23-11    |  NULL |          10 |            NULL |                NULL |
|   10 |    23 | 04-23-12    |    10 |          10 |            NULL |                NULL |
|   11 |    23 | 04-23-13    |  NULL |          10 |              10 |                  10 |
|   12 |    24 | 02-24-10-21 |  NULL |        NULL |            NULL |                NULL |
+------+-------+-------------+-------+-------------+-----------------+---------------------+