True or False

1. In SQL, an aggregate function is a mathematical library that contains functions like sum, min, power, sqrt.
2. If A->B is a functional dependency, then if we know the value of B we can determine the value of A.
3. In a grouping SQL query, the WHERE condition is applied before the grouping of records but the HAVING condition is applied after the forming of groups.
4. The primary key for a record must never be allowed to have a NULL value.
5. A join can be used only once in a query.