Which function is used to adjust the date to the first of the month in calculations?

The function used to adjust the date to the first of the month is DATETRUNC. This function essentially takes a date value and truncates it to the specified date part—such as month, quarter, or year. When you specifically want to find the first day of the month for any given date, you would use DATETRUNC with "month" as the parameter.

For instance, if you have a date like the 15th of March, using DATETRUNC on this date will yield March 1st, effectively providing what you need for analysis at the monthly aggregation level.

The other functions serve different purposes: DATEADD adjusts a date by adding a specified interval (like days, months, or years), DATEPART extracts a component of the date (like the year or month), and DATEDIFF calculates the difference between two dates in specified units. Thus, while these functions are useful in their own right, they do not cater specifically to truncating a date to the first of the month like DATETRUNC does.