Which statement about primary keys is true?
A.
A table can have multiple primary keys.
B.
A primary key accepts one, and only one, NULL value.
C.
A unique index, with matching columns of the primary key, must be created before a primary key can be
created.
D.
A unique bidirectional index is automatically created for the columns of the primary key, if an appropriate
unique index does not already exist.
ifferences between primary key or unique key constraints and unique indexes
It is important to understand that there is no significant difference between a primary unique key constraint and a unique index. The database manager uses a combination of a unique index and the NOT NULL constraint to implement the relational database concept of primary and unique key constraints. Therefore, unique indexes do not enforce primary key constraints by themselves because they allow null values. (Although null values represent unknown values, when it comes to indexing, a null value is treated as being equal to other null values.)
Therefore, if a unique index consists of a single column, only one null value is allowed-more than one null value would violate the unique constraint. Similarly, if a unique index consists of multiple columns, a specific combination of values and nulls can be used only once.
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