first_value¶
This function is used to obtain the value of the first data record in the window corresponding to the current row.
Restrictions¶
The restrictions on using window functions are as follows:
Window functions can be used only in select statements.
Window functions and aggregate functions cannot be nested in window functions.
Window functions cannot be used together with aggregate functions of the same level.
Syntax¶
first_value(<expr>[, <ignore_nulls>]) over ([partition_clause] [orderby_clause] [frame_clause])
Parameters¶
Parameter | Mandatory | Description |
|---|---|---|
expr | Yes | Expression whose return result is to be calculated |
ignore_nulls | No | The value is of the BOOLEAN type, indicating whether to ignore NULL values. The default value is False. If the value is True, the first non-null value in the window is returned. |
partition_clause | No | Partition. Rows with the same value in partition columns are considered to be in the same window. |
orderby_clause | No | It is used to specify how data is sorted in a window. |
frame_clause | No | It is used to determine the data boundary. |
Return Values¶
The return value is of the data type of the parameter.
Example Code¶
Example data
To help you understand how to use functions, this example provides source data and function examples based on the source data. Run the following command to create the logs table and add data:
create table logs(
cookieid string,
createtime string,
url string
)
STORED AS parquet;
Adds the following data:
cookie1 2015-04-10 10:00:02 url2
cookie1 2015-04-10 10:00:00 url1
cookie1 2015-04-10 10:03:04 url3
cookie1 2015-04-10 10:50:05 url6
cookie1 2015-04-10 11:00:00 url7
cookie1 2015-04-10 10:10:00 url4
cookie1 2015-04-10 10:50:01 url5
cookie2 2015-04-10 10:00:02 url22
cookie2 2015-04-10 10:00:00 url11
cookie2 2015-04-10 10:03:04 url33
cookie2 2015-04-10 10:50:05 url66
cookie2 2015-04-10 11:00:00 url77
cookie2 2015-04-10 10:10:00 url44
cookie2 2015-04-10 10:50:01 url55
Example: Groups all records by cookieid, sorts the records by createtime in ascending order, and returns the first row of data in each group. An example command is as follows:
SELECT cookieid, createtime, url,
FIRST_VALUE(url) OVER (PARTITION BY cookieid ORDER BY createtime) AS first
FROM logs;
The command output is as follows:
cookieid createtime url first
cookie1 2015-04-10 10:00:00 url1 url1
cookie1 2015-04-10 10:00:02 url2 url1
cookie1 2015-04-10 10:03:04 url3 url1
cookie1 2015-04-10 10:10:00 url4 url1
cookie1 2015-04-10 10:50:01 url5 url1
cookie1 2015-04-10 10:50:05 url6 url1
cookie1 2015-04-10 11:00:00 url7 url1
cookie2 2015-04-10 10:00:00 url11 url11
cookie2 2015-04-10 10:00:02 url22 url11
cookie2 2015-04-10 10:03:04 url33 url11
cookie2 2015-04-10 10:10:00 url44 url11
cookie2 2015-04-10 10:50:01 url55 url11
cookie2 2015-04-10 10:50:05 url66 url11
cookie2 2015-04-10 11:00:00 url77 url11