Which three are true about scalar subquery expressions?

Scalar subquery expressions are used in Oracle SQL to return a single value from a subquery.

Option C: They can be nested.

Scalar subqueries can indeed be nested within another scalar subquery, provided each subquery returns a single value.

Option D: A scalar subquery expression that returns zero rows evaluates to null.

According to Oracle documentation, if a scalar subquery returns no rows, the result is a NULL value, not zero or any other default.

Option F: They can be used as default values for columns in a create table statement.

Scalar subqueries can be specified in the DEFAULT clause of a column in a CREATE TABLE statement to dynamically assign default values based on the result of the subquery.

Options A, B, and E are incorrect based on Oracle SQL standards and functionalities.