substring String type column by using Add Function Column

[kohry]

#74 is related issue but this is little more specific.

Add Function Column works well with numeric types as intended,
but with string type, text slice is not working.
I think python expression should support string slice, as python is famous for being convenient for text slicing.

(timestamp column in the screenshot above is string type. not datetime)

[kohry]

In this function, I wanted to reformat datetime, so if there is a datetime formatter as a function,
the modeling process will be much simpler.

[shovsj]

1. For now, use sqlite instead of python.
   sqlite support datetime functions. see https://www.sqlite.org/lang_datefunc.html
   For your information, sqlite uses IS0-8601 date and time formats, that is,
   A time string can be in any of the following formats:
   YYYY-MM-DD
   YYYY-MM-DD HH:MM
   YYYY-MM-DD HH:MM:SS
   YYYY-MM-DD HH:MM:SS.SSS
   YYYY-MM-DDTHH:MM
   YYYY-MM-DDTHH:MM:SS
   YYYY-MM-DDTHH:MM:SS.SSS
   HH:MM
   HH:MM:SS
   HH:MM:SS.SSS
   now
   DDDDDDDDDD

strftime function(format, timestring) can be used in this circumstance.

%d		day of month: 00
%f		fractional seconds: SS.SSS
%H		hour: 00-24
%j		day of year: 001-366
%J		Julian day number
%m		month: 01-12
%M		minute: 00-59
%s		seconds since 1970-01-01
%S		seconds: 00-59
%w		day of week 0-6 with Sunday==0
%W		week of year: 00-53
%Y		year: 0000-9999
%%		%

[shovsj]

2. query executor based on sqlite will be developed. one can use not only built-in sql function, aggregate functions but also several udfs and udafs developed by us, or functions developed by herself.