Scalar subquery expressions in Oracle SQL have specific rules:
* Option C: You must enclose them in parentheses.
* Scalar subqueries must be enclosed in parentheses. This is a requirement for syntax clarity and to distinguish the subquery from the rest of the SQL statement.
* Option D: They can return at most one row.
* Scalar subqueries are designed to return exactly one row containing one column. If a scalar subquery returns more than one row, Oracle will throw an error, ensuring that the subquery either returns a single value or no value (NULL).
Options A, B, and E are incorrect based on Oracle SQL functionalities:
* Option A is incorrect because scalar subqueries can indeed be correlated with the parent query.
* Option B is true but not in the context of default constraints for table columns in the CREATE TABLE statement.
* Option E is incorrect because scalar subqueries can only return a single column by definition.