Online Practice Test
SQL Server DBA
A candidate key cannot be regarded as a possible primary key
CORRECT ANSWER : False
The above statement is false. A candidate key can be regarded as a possible primary key. If it is to be regarded as a possible primary key, there are three fundamental demands on the candidate key that we should never deviate from:
1. The candidate key must be unique within its domain.
2. The candidate key cannot hold NULLs (NULL is not zero. Zero is a number. NULL is 'unknown value').
3. The candidate key should never change. It must hold the same value for a given occurrence of an entity for the lifetime of that entity.
A Candidate Key can be any column or a combination of columns that can qualify as unique key in database. There can be multiple Candidate Keys in one table. Each Candidate Key can qualify as Primary Key.
A Primary Key is a column or a combination of columns that uniquely identify a record. Only one Candidate Key can be Primary Key.
For a Candidate Key to qualify as a Primary Key, it should be unique and non-null. So, basically a primary key is just one of the candidate keys, which is just a minimal super key.
Prajakta Pandit 03-10-2017 12:09 AM
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