WINDOW FUNCTION FIRST_VALUE
description
FIRST_VALUE() 返回窗口范围内的第一个值， ignore_null 决定是否忽略 null 值，参数 ignore_null 默认值为 false 。
FIRST_VALUE(expr[, ignore_null]) OVER(partition_by_clause order_by_clause [window_clause])example
我们有如下数据
select id, myday, time_col, state from t;
| id | myday | time_col | state |
|------|-------|-------------|-------|
| 3 | 21 | 04-21-13 | 3 |
| 7 | 22 | 04-22-10-24 | NULL |
| 8 | 22 | 04-22-10-25 | 9 |
| 10 | 23 | 04-23-12 | 10 |
| 4 | 22 | 04-22-10-21 | NULL |
| 9 | 23 | 04-23-11 | NULL |
| 1 | 21 | 04-21-11 | NULL |
| 5 | 22 | 04-22-10-22 | NULL |
| 12 | 24 | 02-24-10-21 | NULL |
| 2 | 21 | 04-21-12 | 2 |
| 6 | 22 | 04-22-10-23 | 5 |
| 11 | 23 | 04-23-13 | NULL |使用 FIRST_VALUE()，根据 myday 分组，返回每个分组中第一个 state 的值：
select * ,
first_value(`state`, 1) over(partition by `myday` order by `time_col` rows between 1 preceding and 1 following) as ignore_null,
first_value(`state`, 0) over(partition by `myday` order by `time_col` rows between 1 preceding and 1 following) as not_ignore_null,
first_value(`state`) over(partition by `myday` order by `time_col` rows between 1 preceding and 1 following) as ignore_null_default
from t 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 |keywords
WINDOW,FUNCTION,FIRST_VALUE