What is the difference between fetch Next and fetch First in the Order By [...] OFFSET [..] FETCH [...] clause?

Question

I executed two sample queries on AdventureWorks2016 and gave me the same result. When should I use NEXT or FIRST keyword then?

select LastName + ' ' + FirstName 
from person.person
order by LastName asc OFFSET 10 rows **Fetch next** 10 rows only

select LastName + ' ' + FirstName
from person.person
order by LastName asc OFFSET 10 rows **Fetch first** 10 rows only

Answer 1

FETCH FIRST and FETCH NEXT do exactly the same thing. The reason both exist because of the preceding OFFSET clause. Using the word FIRST combined with OFFSET can be confusing to a human reader:

SELECT * 
FROM Foo
ORDER BY ID
OFFSET 5 ROWS
FETCH FIRST 5 ROWS ONLY; -- Does this mean rows 6 to 10, or 1 to 5?

Whereas:

SELECT * 
FROM Foo
ORDER BY ID
OFFSET 5 ROWS
FETCH NEXT 5 ROWS ONLY; -- Now it's clear!

SqlFiddle example here.

Answer 2

There is no difference. The SQL standard simply allows both, maybe to bridge differences between syntaxes, or to allow you the choice to write 'fluent English' queries.

This similar as to why the standard allows you to write:

fetch first row
fetch first rows
fetch first 1 row
fetch first 1 rows

(and variants with next) which will all fetch a single row. That this also allows you to write grammatically incorrect sentences like fetch first 2 row is taken for granted.