Actually it depends on your DBMS engine.
Some database management systems will compute twice your expression (once for each comparison), and only once when you use
Actually if the expression can have a non-deterministic result
BETWEEN will have a different behaviour, compare the following in SQLite:
WHERE RANDOM() BETWEEN x AND y -- one random value generated
WHERE RANDOM() >= x AND RANDOM() <= y -- two distinct random values generated
This can be very time consuming if your expression is (for example) a subquery.