Creating a date interval from a single date

Sometimes time intervals are not stored explicitly with a beginning and an end. Instead they are implied by only one field – the change timestamp.

It could be as in the table below where you have currency rates for multiple currencies. Each currency rate change is on its own row; each with a new conversion rate. Also, the table contains rows with empty dates corresponding to the initial conversion rate, before the first change was made.

This table defines a set of non-overlapping intervals, where the begin data is called “Change Date” and the end date is defined by the beginning of the following interval. But since the end date isn’t explicitly stored in a column of its own, we need to create such a column, so that the new table will become a list of intervals.

In this script example the table In_Rates is created by an inline load. Make sure that the dates in the Change Date column are in the same format as the local date format.

Do the following:

1. Determine which time range you want to work with. The beginning of the range must be before the first date in the data and the end of the range must be after the last.

   Let vBeginTime = Num('1/1/2013');
   Let vEndTime = Num('1/3/2013');
   Let vEpsilon = Pow(2,-27);

1. Load the source data, but change empty dates to the beginning of the range defined in the previous bullet. The change date should be loaded as “From Date”.

   Sort the table first according to Currency, then according to the “From Date” descending so that you have the latest dates on top.

   Tmp_Rates:
   LOAD Currency, Rate,
   Date(If(IsNum([Change Date]), [Change Date], $(#vBeginTime))) as FromDate
   Resident In_Rates;

2. Run a second pass through data where you calculate the “To Date”. If the current record has a different currency from the previous record, then it is the first record of a new currency (but its last interval), so you should use the end of the range defined in step 1. If it is the same Currency, you should take the “From Date” from the previous record, subtract a small amount of time, and use this value as “To Date” in the current record.

   Rates:
   LOAD Currency, Rate, FromDate,
   Date(If( Currency=Peek(Currency),
   Peek(FromDate) - $(#vEpsilon),
   $(#vEndTime)
   )) as ToDate
   Resident Tmp_Rates
   Order By Currency, FromDate Desc;

3. Drop the input table and the temporary table.

   Drop Table Tmp_Rates;

The script listed below will update the source table in the following manner:

When the script is run, you will have a table listing the intervals correctly. Use the Preview section of the data model viewer to view the resulting table.

This table can subsequently be used in a comparison with an existing date using the Intervalmatch methods.

Example:  

The entire Qlik Sense script looks like this: