Which four statements are true about constraints on Oracle tables?
A.
A Column can have only one CHECK Constraint.
B.
A NOT NULL Constraint can be defined at the table level.
C.
A UNIQUE constraint permits NULLS.
D.
A PRIMARY KEY Constraint can be added after a table has been created and populated.
E.
A CHECK Constraint can refer to values in other rows.
F.
A UNIQUE Constraint can use a pre-existing index on the constrained column or columns.
G.
A FOREIGN KEY Column can contain NULLS.
The Answer Is:
C, D, F, G
This question includes an explanation.
Explanation:
C: True. A UNIQUE constraint in Oracle SQL allows for the inclusion of NULL values; specifically, it permits multiple NULLs in a column or set of columns but ensures that all non-NULL values are unique.
D: True. A PRIMARY KEY constraint can indeed be added to a table after it has been created and even after it has been populated, as long as the existing data does not violate the primary key constraint rules (i.e., all values must be unique and not NULL).
F: True. A UNIQUE constraint can utilize a pre-existing index on the columns it covers. If a suitable index already exists, Oracle can use this index to enforce the constraint, optimizing performance and resource utilization.
G: True. Columns that are part of a FOREIGN KEY constraint can contain NULL values. This is permissible under SQL standards and Oracle implementation, as a NULL foreign key value is considered to not refer to any row in the referenced table and thus does not violate referential integrity.
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