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

Solution: A relation, in the context of database management systems, is a fundamental structure within the relational data model. It is represented as a table, characterized by having atomic values in each cell, ensuring that each piece of data is indivisible. A relation must also have unique rows (tuples), meaning no two rows are identical. Furthermore, the rows and columns within a relation are considered unordered. The relational data model itself is a way of organizing and representing data and the relationships between different data entities using a collection of these tables, where each table has a distinct set of column names.

Solution: The domain for the 'gender' attribute typically consists of a predefined set of acceptable values. In most common scenarios, the possible domain names for gender are 'Male' and 'Female'. These are the standard, mutually exclusive categories used to classify individuals based on gender.

Solution: Consider the following 'Stock' table:

In this table, the 'Item Code' column uniquely identifies each item. The 'Item Name' column also appears to uniquely identify each item in this specific sample data. Therefore:

Candidate Keys: Attributes or sets of attributes that can uniquely identify a tuple (row) in a relation. In this example, both 'Item Code' and 'Item Name' can serve as candidate keys because each value in these columns is unique and can identify a specific stock item.

Primary Key: One of the candidate keys is selected as the primary key. The primary key is used to uniquely identify each record in the table. In this case, 'Item Code' is typically chosen as the primary key because it's usually a numerical identifier, which is often more efficient and less prone to changes than a descriptive name.

Thus, 'Item Code' is the primary key, and both 'Item Code' and 'Item Name' are candidate keys.

Solution: The terms cardinality and degree are used to describe the dimensions of a table (relation) in a relational database:

• 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 pieces of information are stored for each record.

Let's consider the 'Account' table provided:

Table: Account

The table contains the following columns: 'Acno' and 'Cname'.

The rows shown are: (Ac 100, Sheela), (Ac 101, Darsh), (Ac 102, Kathy).

Based on this example:

Cardinality of Account table: There are 3 rows, so the cardinality is 3.

Degree of Account table: There are 2 columns ('Acno', 'Cname'), so the degree is 2.

Solution: To identify the candidate keys and primary key, we need to find columns or combinations of columns that uniquely identify each row.

• (i) Candidate Keys: A candidate key is an attribute or a set of attributes that can uniquely identify a row. Let's examine the columns:
  • 'Id': Each value in the 'Id' column (101, 104, 105, 109, 103) is unique and can identify a specific transaction record.
  • 'Product': The 'Product' column contains values like 'Plastic Folder', 'Pen Stand Standard', 'Stapler Medium', 'Punching Machine Big', 'Stapler Mini'. In this specific data, each product name is unique. If it's guaranteed that each product name will always be unique for each transaction, then 'Product' could also be a candidate key. However, 'Id' is generally a more reliable unique identifier.
  • Other columns like 'Price', 'Qty', 'Transaction Date' do not uniquely identify rows on their own (e.g., 'Qty' has duplicate values '200' and '100').
  Therefore, the most appropriate columns that can be considered as candidate keys are 'Id' and potentially 'Product' (assuming product names are unique per transaction).
• (ii) Primary Key: The primary key is one of the candidate keys selected to be the main unique identifier for the table. Typically, a simple, non-null, and stable attribute is chosen. In this table, 'Id' is the most suitable choice for the primary key as it is a numerical identifier that is guaranteed to be unique for each transaction record.