| --- |
| { |
| "title": "WINDOW_FUNCTION_FIRST_VALUE", |
| "language": "en" |
| } |
| --- |
| |
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| |
| http://www.apache.org/licenses/LICENSE-2.0 |
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| |
| ## WINDOW FUNCTION FIRST_VALUE |
| ### description |
| |
| FIRST_VALUE() returns the first value in the window's range , ignore_null determines whether to ignore null values , the ignore_null of default value is false . |
| |
| ```sql |
| FIRST_VALUE(expr[, ignore_null]) OVER(partition_by_clause order_by_clause [window_clause]) |
| ``` |
| |
| ### example |
| |
| |
| We have the following data |
| |
| ```sql |
| 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 | |
| ``` |
| |
| Use FIRST_VALUE() to group by myday and return the value of the first state in each group: |
| |
| ```sql |
| 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 |
