Question 14 - Teachoo Sample Paper - Computer Science Class 12 - Solutions to CBSE Sample Paper - Computer Science Class 12

Which of the following statements is FALSE about alternate keys in a relational database? 

1. An alternate key is a candidate key that is not a primary key. 

2. An alternate key can uniquely identify a tuple in a relation. 

3. An alternate key can contain null values. 

4. An alternate key can be a composite key.

Answer:

Answer by student

c. An alternate key can contain null values.

Detailed answer by teachoo

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To answer this question, we need to know what is an alternate key and what are its properties in a relational database. 

• An alternate key is a candidate key that is not a primary key. 
• A candidate key is a minimal set of attributes that can uniquely identify a tuple (row) in a relation (table). 
• A primary key is one of the candidate keys that is chosen to be the main identifier of the tuples.

Let’s go through each of the statements and see why they are true or false.

1. An alternate key is a candidate key that is not a primary key. This statement is TRUE , because it is the definition of an alternate key.
2. An alternate key can uniquely identify a tuple in a relation. This statement is TRUE , because it is one of the properties of an alternate key.
3. An alternate key can contain null values. This statement is FALSE , because null values are not allowed in candidate keys, and hence, not in alternate keys.
4. An alternate key can be a composite key. This statement is TRUE , because an alternate key can be a combination of two or more attributes that can uniquely identify a tuple.

So, the correct answer is c. An alternate key can contain null values.

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