how can i calculate the number of students with age > x in sql?

Answer 1

SELECT COUNT(*) FROM students WHERE age > x

Answer 2

Try:
SELECT COUNT(*) FROM STUDENTS WHERE AGE > x

Answer 3

you can use alias as well for the count(*):
SELECT COUNT(*) AS 'Total Students' FROM students WHERE age > x

Answer 4

SELECT COUNT(*) [TotalStudents] FROM students WHERE age > x