Update query cancelled by user

Question

I have a table that has millions of rows.

Accidentally I wrote an update query over a table without where clause and clicked execute.

It started executing. A

Answer 1

A single update statement will not update some rows. It's all rows or none

This is the atomicity in the ACID properties which SQL server respects well.

Atomicity requires that each transaction is "all or nothing": if one part of the transaction fails, the entire transaction fails, and the database state is left unchanged. An atomic system must guarantee atomicity in each and every situation, including power failures, errors, and crashes.

Then the commit is at the end of the statement, so when you cancel there's no commit