This feature is coming soon!
If you want to compare 2 date periods, it can be difficult using just SQL. You have to modify the dates to they join correctly, and then use date and time logic to get the period you want, dynamically.
The Date Compare Transform makes this so much easier.
Simply pick the time period you want as your base, then pick what time period you want to compare it to.
Grow will then dynamically place your two periods, side by side with all the data, and dynamically update the date range to what you want. This makes it easy to calculate key values or chart this month against last month.