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Database Concepts
CBSE · Class 11 · Informatics Practices
NCERT Solutions for Database Concepts — CBSE Class 11 Informatics Practices.
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EXERCISE
1Give the terms for each of the following:Show solution
A database is a collection of logically related records.
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2Why foreign keys are allowed to have NULL values? Explain with an example.Show solution
A foreign key may be NULL when the referencing record has no related record in the master table, or when the relationship is not applicable.
For example, in the GUARDIAN table, if a guardian does not share a phone number, then GPhone can be NULL. Similarly, a foreign key in another table can be NULL if that row does not yet refer to any record in the related table.
For example, in the GUARDIAN table, if a guardian does not share a phone number, then GPhone can be NULL. Similarly, a foreign key in another table can be NULL if that row does not yet refer to any record in the related table.
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3Differentiate between:Show solution
### Database state and database schema
- Database schema: the design or skeleton of the database. It shows table names, columns, data types, constraints, and relationships.
- Database state: the current data stored in the database at a particular time; also called an instance.
### Primary key and foreign key
- Primary key: the attribute or set of attributes that uniquely identifies each tuple in its own relation.
- Foreign key: an attribute whose value is taken from the primary key of another relation to create a link between tables.
### Degree and cardinality of a relation
- Degree: number of attributes/columns in a relation.
- Cardinality: number of tuples/rows in a relation.
- Database schema: the design or skeleton of the database. It shows table names, columns, data types, constraints, and relationships.
- Database state: the current data stored in the database at a particular time; also called an instance.
### Primary key and foreign key
- Primary key: the attribute or set of attributes that uniquely identifies each tuple in its own relation.
- Foreign key: an attribute whose value is taken from the primary key of another relation to create a link between tables.
### Degree and cardinality of a relation
- Degree: number of attributes/columns in a relation.
- Cardinality: number of tuples/rows in a relation.
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4Compared to a file system, how does a database management system avoid redundancy in data through a database?Show solution
Compared to a file system, a DBMS reduces redundancy by storing related data only once and linking tables through common fields.
For example, in the school database:
- SName is not stored in the ATTENDANCE table because it already exists in STUDENT.
- Guardian details are separated into a GUARDIAN table so the same guardian data is not repeated for siblings.
Thus, a DBMS avoids repeated storage of the same data and reduces data redundancy.
For example, in the school database:
- SName is not stored in the ATTENDANCE table because it already exists in STUDENT.
- Guardian details are separated into a GUARDIAN table so the same guardian data is not repeated for siblings.
Thus, a DBMS avoids repeated storage of the same data and reduces data redundancy.
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5What are the limitations of file system that can be overcome by a relational DBMS?Show solution
The limitations of a file system that can be overcome by a relational DBMS are:
1. Difficulty in access
2. Data redundancy
3. Data inconsistency
4. Data isolation
5. Data dependence
6. Controlled data sharing
A DBMS also makes it easier to store, update, delete, and retrieve data efficiently.
1. Difficulty in access
2. Data redundancy
3. Data inconsistency
4. Data isolation
5. Data dependence
6. Controlled data sharing
A DBMS also makes it easier to store, update, delete, and retrieve data efficiently.
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6A school has a rule that each student must participate in a sports activity. So each one should give only one preference for sports activity. Suppose there are five students in a class, each having a unique roll number. The class representative has prepared a list of sports preferences as shown below. Answer the following:Show solution
The table shown has issues because:
- Roll_no 17 appears twice with different preferences.
- Roll_no NULL appears with Kabaddi.
- Roll_no 24 has preference as NULL.
These cases are used in the following sub-questions.
- Roll_no 17 appears twice with different preferences.
- Roll_no NULL appears with Kabaddi.
- Roll_no 24 has preference as NULL.
These cases are used in the following sub-questions.
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6aRoll no 24 may not be interested in sports. Can a NULL value be assigned to that student's preference field?Show solution
Yes. If the student is not interested in sports, the preference field can be NULL because it represents an unknown or non-applicable value.
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6bRoll no 17 has given two preferences sports. Which property of relational DBMC is violated here? Can we use any constraint or key in the relational DBMS to check against such violation, if any?Show solution
The property violated is Property 2 of relations: each tuple in a relation must be distinct.
Since roll no 17 has two different preferences, it means the student has more than one value for the same field, which is not allowed if the rule is only one preference per student.
To prevent this, the Roll_no field can be made UNIQUE and NOT NULL. If one row per student is required, then Roll_no should act as the key. However, the text mainly suggests using constraints like UNIQUE and NOT NULL to ensure each roll number appears only once.
Since roll no 17 has two different preferences, it means the student has more than one value for the same field, which is not allowed if the rule is only one preference per student.
To prevent this, the Roll_no field can be made UNIQUE and NOT NULL. If one row per student is required, then Roll_no should act as the key. However, the text mainly suggests using constraints like UNIQUE and NOT NULL to ensure each roll number appears only once.
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6cKabaddi was not chosen by any student. Is it possible to have this tuple in the Sports Preferences relation?Show solution
Yes. A tuple like (NULL, Kabaddi) can exist if the roll number is unknown or not given, because NULL is allowed for a value that is unknown or non-applicable. But if Roll_no is meant to be the primary key, then it cannot be NULL.
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7In another class having 2 sections, the two respective class representatives have prepared 2 separate Sports Preferences tables, as shown below:Show solution
Yes, the two relations are equivalent in state because they contain the same set of tuples, only the order of rows and order of columns is different.
In relational model, the sequence of attributes is immaterial and the sequence of tuples is immaterial. So both tables represent the same relation.
In relational model, the sequence of attributes is immaterial and the sequence of tuples is immaterial. So both tables represent the same relation.
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8The school canteen wants to maintain records of items available in the school canteen and generate bills when students purchase any item from the canteen. The school wants to create a canteen database to keep track of items in the canteen and the items purchased by students. Design a database by answering the following questions:Show solution
A suitable design is:
- ITEM relation to store item details such as item name and price.
- BILL / PURCHASE relation to store purchased items and quantity.
The database should ensure that:
- each item is stored only once,
- bills are generated only for available items,
- and the same bill is not generated for different orders.
- ITEM relation to store item details such as item name and price.
- BILL / PURCHASE relation to store purchased items and quantity.
The database should ensure that:
- each item is stored only once,
- bills are generated only for available items,
- and the same bill is not generated for different orders.
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8aTo store each item name along with its price, what relation should be used? Decide appropriate attribute names along with their data type. Each item and its price should be stored only once. What restriction should be used while defining the relation?Show solution
Use an ITEM relation.
A suitable structure can be:
- ItemID: integer
- ItemName: string
- Price: decimal/integer
Example: ITEM(ItemID, ItemName, Price)
To store each item only once, use UNIQUE on ItemName or make ItemID the primary key. Since each item should be stored once, the important restriction is UNIQUE for item identity, along with a PRIMARY KEY.
A suitable structure can be:
- ItemID: integer
- ItemName: string
- Price: decimal/integer
Example: ITEM(ItemID, ItemName, Price)
To store each item only once, use UNIQUE on ItemName or make ItemID the primary key. Since each item should be stored once, the important restriction is UNIQUE for item identity, along with a PRIMARY KEY.
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8bIn order to generate bill, we should know the quantity of an item purchased. Should this information be in a new relation or a part of the previous relation? If a new relation is required, decide appropriate name and data type for attributes. Also, identify appropriate primary key and foreign key so that the following two restrictions are satisfied:Show solution
This information should be in a new relation, because bill details are separate from item details.
A suitable relation can be:
- BILL or PURCHASE
- Attributes: BillNo, ItemID, Quantity
Example structure: PURCHASE(BillNo, ItemID, Quantity)
- Primary key: BillNo if each bill is unique, or a composite key like (BillNo, ItemID) if one bill can contain multiple items.
- Foreign key: ItemID referencing the ITEM relation.
This satisfies:
1. The same bill cannot be generated for different orders → use BillNo as unique/primary key.
2. Bill can be generated only for available items → ItemID must match an existing item in ITEM.
A suitable relation can be:
- BILL or PURCHASE
- Attributes: BillNo, ItemID, Quantity
Example structure: PURCHASE(BillNo, ItemID, Quantity)
- Primary key: BillNo if each bill is unique, or a composite key like (BillNo, ItemID) if one bill can contain multiple items.
- Foreign key: ItemID referencing the ITEM relation.
This satisfies:
1. The same bill cannot be generated for different orders → use BillNo as unique/primary key.
2. Bill can be generated only for available items → ItemID must match an existing item in ITEM.
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8cThe school wants to find out how many calories students intake when they order an item. In which relation should the attribute 'calories' be stored?Show solution
The attribute calories should be stored in the ITEM relation, because calories are a property of the item itself, not of the purchase transaction.
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9An organisation wants to create a database EMP-DEPENDENT to maintain following details about its employees and their dependent.Show solution
The database is:
- EMPLOYEE(AadharNumber, Name, Address, Department, EmployeeID)
- DEPENDENT(EmployeeID, DependentName, Relationship)
It is meant to store employee details and their dependent details.
- EMPLOYEE(AadharNumber, Name, Address, Department, EmployeeID)
- DEPENDENT(EmployeeID, DependentName, Relationship)
It is meant to store employee details and their dependent details.
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9aName the attributes of EMPLOYEE, which can be used as candidate keys.Show solution
The attributes of EMPLOYEE that can be used as candidate keys are AadharNumber and EmployeeID, because both are unique identifiers.
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9bThe company wants to retrieve details of dependent of a particular employee. Name the tables and the key which are required to retrieve this detail.Show solution
To retrieve dependent details of a particular employee, use the tables EMPLOYEE and DEPENDENT.
The required key is EmployeeID, which acts as a primary key in EMPLOYEE and a foreign key in DEPENDENT.
The required key is EmployeeID, which acts as a primary key in EMPLOYEE and a foreign key in DEPENDENT.
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9cWhat is the degree of EMPLOYEE and DEPENDENT relation?Show solution
- EMPLOYEE(AadharNumber, Name, Address, Department, EmployeeID) has 5 attributes, so its degree = 5.
- DEPENDENT(EmployeeID, DependentName, Relationship) has 3 attributes, so its degree = 3.
- DEPENDENT(EmployeeID, DependentName, Relationship) has 3 attributes, so its degree = 3.
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10School uniform is available at M/s Sheetal Private Limited. They have maintained SCHOOL_UNIFORM Database with two relations viz. UNIFORM and COST. The following figure shows database schema and its state.
10aCan they insert the following tuples to the UNIFORM Relation? Give reasons in support of your answer.
10bCan they insert the following tuples to the COST Relation? Give reasons in support of your answer.
11In a multiplex, movies are screened in different auditoriums. One movie can be shown in more than one auditorium. In order to maintain the record of movies, the multiplex maintains a relational database consisting of two relations viz. MOVIE and AUDI respectively as shown below:
11aIs it correct to assign Movie_ID as the primary key in the MOVIE relation? If no, then suggest an appropriate primary key.
11bIs it correct to assign AudiNo as the primary key in the AUDI relation? If no, then suggest appropriate primary key.
11cIs there any foreign key in any of these relations?
12For the above given database STUDENT-PROJECT, answer the following:
12aName primary key of each table.
12bFind foreign key(s) in table PROJECT-ASSIGNED.
12cIs there any alternate key in table STUDENT? Give justification for your answer.
12dCan a user assign duplicate value to the field RollNo of STUDENT table? Jusify.
13For the above given database STUDENT-PROJECT, can we perform the following operations?
13aInsert a student record with missing roll number value.
13bInsert a student record with missing registration number value.
13cInsert a project detail without submission-date.
13dInsert a record with registration ID IP-101-19 and ProjectNo 206 in table PROJECT-ASSIGNED.
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Sources & Official References
- NCERT Official — ncert.nic.in
- CBSE Academic — cbseacademic.nic.in
- CBSE Official — cbse.gov.in
- National Education Policy 2020 — education.gov.in
Content is aligned to the official syllabus. Refer to the board website for the latest curriculum.
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