Comment to Exam Question 3a

Request: 

One of the subqueries has

SELECT T1.A
FROM T1, T2
WHERE T1.A = T2.B AND T2.C > T.X

how can we compare T2.C > T.X​ when we have only T1 and T2 in the FROM clause?

Answer:

We have (briefly) considered this case in the course.
In this case, the value of X is taken from the main query (i.e., from the outside of the subquery).