E . True. The deptno column in the emp table can contain null values unless explicitly constrained not to, which is not the case here.
F . True. The three statements can execute successfully in the order shown. The foreign key constraint in the dept table is referring to the emp table which does not yet exist, but this is not a problem because it is not immediately enabled. The emp table is created next with a disabled foreign key constraint referencing the already created dept table. Finally, the foreign key constraint on emp is enabled after both tables are in place.
A is incorrect because the MGR column can contain NULL values as there is no NOT NULL constraint applied to it. B is incorrect because the CREATE TABLE EMP statement contains a disabled foreign key, so it can be created before CREATE TABLE DEPT. C and D are incorrect because foreign key constraints do not need to be removed and added later for the tables to be created successfully; they can be managed as shown in the provided statements.