What is -1 and 0 doing in date add function? - sql

Question

dateadd(mm, DATEPART(MONTH, DATE) - 1, 0) + DATEPART(DAY, DATE) - 1
OUTPUT date is in the year for example 1990-12-02 00:00:00:000
Full query is below:
SELECT dateadd(yy, (
DATEPART(YEAR, GETDATE()) + (
CASE
WHEN DATEPART(MONTH, GP_DATE) > 10
THEN 0
ELSE 1
END
) - 1900
), 0) + dateadd(mm, DATEPART(MONTH, GP_DATE) - 1, 0) + DATEPART(DAY, GP_DATE) - 1 GP_DATE
from table
I am trying to convert this query into snowflake syntax and snowflake syntax dateadd function does not allow 1, 0.

Answer 1

In SQL Server, there are some rather ugly implicit conversions available between datetimes and integers.
0, when converted to a date, becomes 1900-01-01.
You're also allowed to do maths on dates. Adding or subtracting 1 adds or subtracts 1 day from the date. Putting these facts together, we have:
dateadd(yy, (
DATEPART(YEAR, GETDATE()) + (
CASE
WHEN DATEPART(MONTH, GP_DATE) > 10
THEN 0
ELSE 1
END
) - 1900
), 0)
Which is taking the current year, and subtracting 1900 from it (or 1899 if the month is less than 11, for whatever reason). We then take that number and add it back to the date 0 (which as stated above is 1900-01-01). The result is that we get the first of January of next year or this year, depending on the month of GP_DATE. Call this D1.
Moving on:
dateadd(mm, DATEPART(MONTH, GP_DATE) - 1, 0)
is taking the month of GP_DATE, subtracting 1 from it and adding that number of months to the date 0 (1900-01-01). The result is a the 1st of whichever month GP_DATE is in, but in 1900. Let's call this D2.
When we add D1 and D2 together, we approximately get a date of the 1st of whichever month GP_DATE is in, in either this year or next year. Note, however, that this goes wrong if the D1 year is a leap year, we get it wrong by a day for months after February.
Finally, we take DATEPART(DAY, GP_DATE) - 1, where we take the day of the month from GP_DATE, subtract 1, and add that on to our result so far. This should set the final date to be on the same day of the month as GP_DATE, except for the error mentioned above.
So, it appears that the code is trying to take GP_DATE and get the same date in either this year or next year, depending on how late in the year GP_DATE is. However, it also appears it was never tested with consideration for leap years.
A far more likely correct version of this query would be this instead:
SELECT
DATEADD(year,
DATEDIFF(year,GP_DATE,CURRENT_TIMESTAMP) +
CASE WHEN DATEPART(month,GP_DATE)>10 THEN 0 ELSE 1 END
,GP_DATE)