CBSE Class 12 Computer Science Chapter 11: Database Concepts NCERT Solutions

A relation is a fundamental structure in the relational data model, essentially representing a table. It is characterized by having atomic values (indivisible data) in each of its cells, unique rows (also known as tuples), and unordered rows and columns. The relational data model organizes and represents data, along with the relationships between different pieces of data, through a collection of these tables, where each table is called a relation. Each relation has a set of columns, and each column has a unique name within that relation.

The domain names possible for the 'gender' attribute typically represent the distinct categories of gender. In most common database contexts, these would be:

• Male
• Female

These are the specific, allowed values that can be entered into the gender column for any record.

A table may contain one or more attributes, or combinations of attributes, that can uniquely identify each row (tuple). Such attributes are called Candidate Keys. From these candidate keys, one is selected to serve as the Primary Key, which is the main identifier for the rows in the table.

Let's consider the 'Stock' table provided:

In this 'Stock' table, the column 'Candidates Keys' (which seems to be intended as 'Item Code' or 'Stock ID') uniquely identifies each item. If 'Item Name' were also guaranteed to be unique, it could also be a candidate key. Assuming 'Candidates Keys' is the unique identifier, it would be the Primary Key. If 'Item Name' could potentially have duplicates (e.g., multiple types of 'Pen'), then only 'Candidates Keys' would be a candidate key.

Illustration:

• Candidate Keys: If 'Candidates Keys' (Item Code) and 'Primary Keys' (Item Name) are both unique, they are both candidate keys.
• Primary Key: 'Candidates Keys' (Item Code) is selected as the Primary Key because it's a numerical code, often preferred for efficiency.

The difference between cardinality and degree of a table is as follows:

• Cardinality: This refers to the number of rows (or tuples) present in a table. It indicates how many records are stored in the table.
• Degree: This refers to the number of columns (or attributes) present in a table. It indicates how many different types of data are stored for each record.

Example: Consider the 'Account' table:

In the 'Account' table:

• The Cardinality is 3, because there are 3 rows (Sheela, Darsh, Kathy).
• The Degree is 2, because there are 2 columns (Acno, Cname).

To identify the candidate keys and primary key, we need to find columns that can uniquely identify each row.

• (i) Candidate Keys: A candidate key is an attribute or set of attributes that can uniquely identify each tuple in a relation. Looking at the table:
  • The 'Id' column contains unique numerical values (101, 104, 105, 109, 103) for each row. Thus, 'Id' can be a candidate key.
  • The 'Product' column contains names like 'Plastic Folder', 'Pen Stand Standard', 'Stapler Medium', etc. Assuming these product descriptions are unique for each entry in this specific table, 'Product' could also potentially serve as a candidate key. However, in real-world scenarios, product names might not always be unique (e.g., different types of 'Stapler'). Based on the provided data, 'Product' appears unique.
  Therefore, the most appropriate columns that can be considered as candidate keys are 'Id' and 'Product'.
• (ii) Primary Key: The primary key is chosen from the set of candidate keys. It is the key that the database designer selects as the main identifier for the table. Typically, a simple, single-column key like 'Id' is preferred for efficiency and simplicity. In this table, 'Id' is a strong candidate for the primary key as it is a numerical identifier that is guaranteed to be unique for each record.

Conclusion:

• Candidate Keys: Id, Product
• Primary Key: Id