• If an input value is missing or null, it is not factored in the computation. For example, for the first row in the dataset, the rolling count of distinct values of previous values is undefined.

• The row from which to extract a value is determined by the order in which the rows are organized based on the order parameter. If you are working on a randomly generated sample of your dataset, the values that you see for this function might not correspond to the values that are generated on the full dataset during job execution.

• The function takes a column name and three optional integer parameters that determine the maximum number of values and the window backward and forward of the current row.
  • By default, the list is limited to 1000 values. To change the maximum number of values, specify a value for the limit parameter.
    
  • For the window parameters, the default values are -1 and 0, which computes the rolling function from the current row back to the first row of the dataset.
• This function works with the following transforms:
  • Window Transform
  • Set Transform
  • Derive Transform

For more information on a non-rolling version of this function, see LIST Function.

Column example:

Rows before example:

Output: Generates the new column, which contains the rolling list of values of the current row and the twenty previous row values in the myNumber column, with a limit of 5 total values.

Rows before and after example:

Output: Generates the new column, which contains the rolling list of values from the previous 299 rows, the current row value, and the 200 rows after the current one in the myNumber column, with a limit of 20 total values.     

For more information on the order and group parameters, see Window Transform.

col_ref

Name of the column whose values are used to compute the function. 

• Multiple columns and wildcards are not supported.

limit_int

Non-negative integer that defines the maximum number of values to extract into the list array. 

rowsBefore_integer, rowsAfter_integer

Integers representing the number of rows before or after the current one from which to compute the rolling function, including the current row. For example, if the first value is 5, the current row and the four rows after it are used in the computation. Negative values for k compute the rolling average from rows preceding the current one.

• rowBefore=1 generates the current row value only.
• rowBefore=-1  uses all rows preceding the current one.
• If rowsAfter is not specified, then the value 0 is applied.
• If a group parameter is applied, then these parameter values should be no more than the maximum number of rows in the groups.

Example - Recent Finishers by Boat Type

The following dataset includes the finishing times for each boat in a race. As part of the race, each boat may be assigned one or more penalties in terms of seconds. So, the total time for the race is computed by adding two columns. 

You are interested in the list of recent finishers by each finishing time.

Source:

Transform:

Compute the total time for each racer by summing the raceTime_secs column and the racePenalties_secs column: