A . False. Constraints are enforced during INSERT and UPDATE operations, and by the nature of their definition, they impact DELETE operations as well (in the case of referential constraints).
B . False. A column with a foreign key constraint can contain a NULL value unless it is also constrained to be NOT NULL.
C . False. Not all constraints can be defined at the column level. For example, some constraints such as FOREIGN KEY constraints are more commonly defined at the table level.
D . True. A constraint can be disabled regardless of whether the constrained column contains data. However, re-enabling the constraint requires that all data satisfy the constraint rules.
E . True. A column with a UNIQUE constraint can indeed contain a NULL value, as NULL is considered not equal to any value, including itself. This means that multiple rows with NULL values do not violate the UNIQUE constraint.
Reference:
Oracle Documentation on Constraints: https://docs.oracle.com/database/121/SQLRF/clauses002.htm#SQLRF52271 Oracle Documentation on Enabling and Disabling Constraints: https://docs.oracle.com/database/121/ADMIN/clustrct.htm#ADMIN13508