Why are these two SQL date functions not providing the same answer - tsql

Question

I've actually already solved the problem, but I'm trying to understand why the problem occurred because as far as I can see it has no reason to happen.
I have a rather large query that I run to prepare a table with some often used combinations. Generally, it only contains 2 years of data. Occasionally I will reconstruct it. While doing this I tweaked the query to add more information, but suddenly the result no longer matched up to the old query. Comparing the old to the new I noticed several missing orders. Amazingly, even after removing the tweaked parts the results still didn't match up.
I ultimately tracked the problem down to my WHERE clause, which was different from how I did it last time.
The type of the orderdate column I go over has type (datetime, null)
One of the orders that was omitted had this as date:
2018-12-23 20:58:52.383
An order that was included had this as date:
2019-01-28 15:20:49.107
It looks exactly the same to me.
The entire query is the same, except for the WHERE clause. My original where was:
WHERE DATEPART(yyyy,tbOrder.[OrderDate]) >= DATEPART(yyyy,GETDATE()-2)
My new where is now:
WHERE tborder.[OrderDate] >= DATEADD(yy, DATEDIFF(yy, 0, GETDATE())-2, 0)
Any help in understanding why the original where clause drops some lines would be greatly appreciated.

Answer 1

Because you are doing two different things. First predicate,
WHERE DATEPART(yyyy,tbOrder.[OrderDate]) >= DATEPART(yyyy,GETDATE()-2)
Take all order dates that are bigger than the year for the day before yesterday or two days before. Notice that, -2 is inside the brackets.
Second predicate,
WHERE tborder.[OrderDate] >= DATEADD( yy, DATEDIFF( yy, 0, GETDATE() ) - 2, 0)
Take all order dates bigger than two years before, i.e., datediff(yy,startdate,enddate) will return the year result of the difference between today and the initial value for date datatype, which is 1900-01-01. Then, add this, -2, to 1900-01-01. The second expression is the form of:
1900 + ( 201X - 1898 )
I simplified 1900 - 2 = 1898.

Answer 2

The two expressions return completely different things, so it shouldn't be a surprise the results are different. The first one returns the current year as a number (or the year of the day before yesterday to be precise). The second one returns January 1st two years ago.
You can put both expressions in a SELECT query to see what they return :
select DATEPART(yyyy,GETDATE()-2), DATEADD(yy, DATEDIFF(yy, 0, GETDATE()) - 2, 0)
The result is :
2019 2017-01-01 00:00:00.000
Both expressions are more complex than they need to be. The first condition will also harm performance because DATEPART(yyyy,tbOrder.[OrderDate]) prevents the server from using any indexes that cover OrderDate.
The question doesn't explain what you actually want to return. If you wanted to return all rows in the current year you can use :
Where
OrderDate >=DATEFROMPARTS( YEAR(GETDATE()) ,1,1) and
OrderDate < DATEFROMPARTS( YEAR(GETDATE()) + 1,1,1)
The same can be used to find rows two years in the past :
Where
OrderDate >= DATEFROMPARTS( YEAR(GETDATE()) -2 ,1,1) and
OrderDate < DATEFROMPARTS(YEAR(GETDATE()) - 1,1,1)
All rows since January 1st two years ago :
Where OrderDate >= DATEFROMPARTS( YEAR(GETDATE()) -2 ,1,1)
All those queries can take advantage of indexes that cover OrderDate.
Date range queries become a lot easier if you use a Calendar table. A Calendar table is a table that contains eg 50 or 100 years' worth of dates with extra columns for month, month day, week number, day of week, quarter, semester, month and day names, holidays, business reprorint periods, formatted short, long dates etc.
This makes yearly, monthly or weekly queries as easy as joining with the Calendar table and filtering based on the month or period you want.
In this case, retrieving rows two yeas in the past would look like :
From Orders inner Join Calendar on OrderDate=Calendar.Date
Where Calendar.Year=YEAR(GETDATE())-2
That may not looks so impressive but what about Q2 two years ago?
From Orders inner Join Calendar on OrderDate=Calendar.Date
Where Calendar.Year=YEAR(GETDATE())-2 and Quarter=2
Two years ago, same quarter
From Orders inner Join Calendar on OrderDate=Calendar.Date
Where Calendar.Year=YEAR(GETDATE())-2 and Quarter=DATEPART(q,GETDATE())
Retrieving totals for the current quarter for the last two years :
SELECT Year,Quarter,SUM(Total) QuarterTotal
From Orders inner Join Calendar on OrderDate=Calendar.Date
Where Calendar.Year > YEAR(GETDATE())-2 and Quarter=DATEPART(q,GETDATE())
GROUP BY Calendar.Year