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Updated on 2023-12-21 GMT+08:00
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lag

This function is used to return the value of the nth row upwards within a specified window.

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

lag(<expr>[, bigint <offset>[, <default>]]) over([partition_clause] orderby_clause)

Parameters

Table 1 Parameters

Parameter

Mandatory

Description

expr

Yes

Expression whose return result is to be calculated

offset

No

Offset. It is a constant of the BIGINT type and its value is greater than or equal to 0. The value 0 indicates the current row, the value 1 indicates the previous row, and so on. The default value is 1. If the input value is of the STRING or DOUBLE type, it is implicitly converted to the BIGINT type before calculation.

default

Yes

Constant. The default value is NULL.

Default value when the range specified by offset is out of range. The value must be the same as the data type corresponding to expr. If expr is non-constant, the evaluation is performed based on the current row.

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.

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

Groups all records by cookieid, sorts the records by createtime in ascending order, and returns the value of the second row above the window. An example command is as follows:

Example 1:

SELECT cookieid, createtime, url,
       LAG(createtime, 2) OVER (PARTITION BY cookieid ORDER BY createtime) AS last_2_time
FROM logs;
-- Returned result:
cookieid createtime         url  last_2_time
cookie1 2015-04-10 10:00:00 url1 NULL
cookie1 2015-04-10 10:00:02 url2 NULL
cookie1 2015-04-10 10:03:04 url3 2015-04-10 10:00:00
cookie1 2015-04-10 10:10:00 url4 2015-04-10 10:00:02
cookie1 2015-04-10 10:50:01 url5 2015-04-10 10:03:04
cookie1 2015-04-10 10:50:05 url6 2015-04-10 10:10:00
cookie1 2015-04-10 11:00:00 url7 2015-04-10 10:50:01
cookie2 2015-04-10 10:00:00 url11 NULL
cookie2 2015-04-10 10:00:02 url22 NULL
cookie2 2015-04-10 10:03:04 url33 2015-04-10 10:00:00
cookie2 2015-04-10 10:10:00 url44 2015-04-10 10:00:02
cookie2 2015-04-10 10:50:01 url55 2015-04-10 10:03:04
cookie2 2015-04-10 10:50:05 url66 2015-04-10 10:10:00
cookie2 2015-04-10 11:00:00 url77 2015-04-10 10:50:01

Note: Because no default value is set, NULL is returned when the preceding two rows do not exist.

Example 2:

SELECT cookieid, createtime, url,
       LAG(createtime,1,'1970-01-01 00:00:00') OVER (PARTITION BY cookieid ORDER BY createtime) AS last_1_time
FROM cookie4;
-- Result:
cookieid createtime          url last_1_time
cookie1 2015-04-10 10:00:00 url1 1970-01-01 00:00:00 (The default value is displayed.)
cookie1 2015-04-10 10:00:02 url2 2015-04-10 10:00:00
cookie1 2015-04-10 10:03:04 url3 2015-04-10 10:00:02
cookie1 2015-04-10 10:10:00 url4 2015-04-10 10:03:04
cookie1 2015-04-10 10:50:01 url5 2015-04-10 10:10:00
cookie1 2015-04-10 10:50:05 url6 2015-04-10 10:50:01
cookie1 2015-04-10 11:00:00 url7 2015-04-10 10:50:05
cookie2 2015-04-10 10:00:00 url11 1970-01-01 00:00:00 (The default value is displayed.)
cookie2 2015-04-10 10:00:02 url22 2015-04-10 10:00:00
cookie2 2015-04-10 10:03:04 url33 2015-04-10 10:00:02
cookie2 2015-04-10 10:10:00 url44 2015-04-10 10:03:04
cookie2 2015-04-10 10:50:01 url55 2015-04-10 10:10:00
cookie2 2015-04-10 10:50:05 url66 2015-04-10 10:50:01
cookie2 2015-04-10 11:00:00 url77 2015-04-10 10:50:05
Parent topic: Window Functions