The efficiency of file processing in Linux environment has a strong positive correlation to how large your brain capacity is to remember common commands. So I write down commands to release brain space.

Intersection of records:

grep -F -f a.txt b.txt | sort | uniq

Subtraction of records:

grep -F -v -f a.txt b.txt | sort | uniq
# or:
grep -F -v -f b.txt a.txt | sort | uniq