Integrity constraints in DBMS Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are integrity constraints in a database?

Integrity constraints are rules that ensure the accuracy and consistency of data in a database. They prevent invalid data from being entered.

beginner

Name the four main types of integrity constraints.

The four main types are: 1) Entity Integrity, 2) Referential Integrity, 3) Domain Integrity, and 4) User-Defined Integrity.

beginner

What is Entity Integrity?

Entity Integrity ensures that each row in a table has a unique and non-null primary key, so every record can be uniquely identified.

intermediate

Explain Referential Integrity with a real-life example.

Referential Integrity means that a foreign key in one table must match a primary key in another table or be null. For example, a student's course enrollment must refer to an existing course.

beginner

What is Domain Integrity?

Domain Integrity restricts the type, format, or range of values that can be stored in a column, like allowing only dates or positive numbers.

Which constraint ensures that a primary key cannot be null?

A Entity Integrity

B Referential Integrity

C Domain Integrity

D User-Defined Integrity

Entity Integrity requires that primary keys are unique and not null.

What does Referential Integrity enforce?

A Primary keys must be unique

B Data types must be consistent

C Foreign keys must match primary keys or be null

D User permissions on data

Referential Integrity ensures foreign keys refer to existing primary keys or are null.

Which constraint restricts the allowed values in a column?

A Entity Integrity

B Domain Integrity

C Referential Integrity

D User-Defined Integrity

Domain Integrity controls the type and range of data allowed in a column.

User-Defined Integrity constraints are:

A Rules defined by users for specific business needs

B Only related to foreign keys

C Automatically enforced by the database engine

D Standard rules like primary key uniqueness

User-Defined Integrity allows custom rules based on business logic.

What happens if an integrity constraint is violated?

A The database ignores the violation

B The database deletes the entire table

C The database automatically fixes the data

D The database rejects the data operation

Violating an integrity constraint causes the database to reject the operation to keep data consistent.

Describe the four main types of integrity constraints and their purpose.

Think about how each rule helps keep data accurate and consistent.

Explain with an example how Referential Integrity works in a database.

Consider how tables relate to each other in real life.

Practice

1. Which of the following best describes the purpose of integrity constraints in a database?

easy

A. To ensure data accuracy and consistency

B. To speed up database queries

C. To store large amounts of data

D. To create user interfaces

Solution

1. Step 1: Understand the role of integrity constraints

   Integrity constraints are rules that keep data accurate and consistent in a database.

2. Step 2: Match the purpose with the options

   Only To ensure data accuracy and consistency mentions accuracy and consistency, which matches the purpose of integrity constraints.

3. Final Answer:

   To ensure data accuracy and consistency -> Option A

4. Quick Check:

   Integrity constraints = data accuracy and consistency [OK]

Hint: Integrity constraints keep data correct and reliable [OK]

Common Mistakes:

• Confusing constraints with performance features
• Thinking constraints create user interfaces
• Assuming constraints store data

2. Which of the following is the correct syntax to declare a NOT NULL constraint on a column named email in SQL?

easy

A. email VARCHAR(50) NOT NULL

B. email VARCHAR(50) NULL

C. email VARCHAR(50) UNIQUE

D. email VARCHAR(50) PRIMARY KEY

Solution

1. Step 1: Recall the syntax for NOT NULL constraint

   The NOT NULL constraint is added after the data type to ensure the column cannot have empty values.

2. Step 2: Identify the correct option

   email VARCHAR(50) NOT NULL correctly uses email VARCHAR(50) NOT NULL. Other options either allow NULL or use different constraints.

3. Final Answer:

   email VARCHAR(50) NOT NULL -> Option A

4. Quick Check:

   NOT NULL syntax = column datatype NOT NULL [OK]

Hint: NOT NULL means column must have a value, no blanks allowed [OK]

Common Mistakes:

• Using NULL instead of NOT NULL
• Confusing UNIQUE with NOT NULL
• Using PRIMARY KEY when only NOT NULL is needed

3. Consider this SQL table definition:
<pre>CREATE TABLE Students ( ID INT PRIMARY KEY, Name VARCHAR(100) NOT NULL, Age INT CHECK (Age >= 18) );
What will happen if you try to insert (1, 'Alice', 16) into this table?

medium

A. Insertion succeeds without error

B. Insertion fails due to CHECK constraint violation

C. Insertion fails due to PRIMARY KEY violation

D. Insertion fails due to NOT NULL violation

Solution

1. Step 1: Analyze the CHECK constraint on Age

   The CHECK constraint requires Age to be 18 or older.

2. Step 2: Compare the inserted Age value

   The inserted Age is 16, which violates the CHECK constraint.

3. Final Answer:

   Insertion fails due to CHECK constraint violation -> Option B

4. Quick Check:

   Age 16 violates CHECK (Age >= 18) [OK]

Hint: CHECK constraints validate data ranges or conditions [OK]

Common Mistakes:

• Ignoring CHECK constraints and assuming insert works
• Confusing PRIMARY KEY violation with CHECK violation
• Thinking NOT NULL is violated when it is not

4. You have a table with a UNIQUE constraint on the username column. Assume the table already contains a row with username = 'john_doe'. Which of the following SQL statements will cause an error?

medium

A. INSERT INTO Users (username) VALUES ('new_user');

B. INSERT INTO Users (username) VALUES ('jane_smith');

C. INSERT INTO Users (username) VALUES ('john_doe');

D. UPDATE Users SET username = 'jane_smith' WHERE id = 1;

Solution

1. Step 1: Understand UNIQUE constraint behavior

   The UNIQUE constraint prevents duplicate values in the username column.

2. Step 2: Identify duplicate insertion

   INSERT INTO Users (username) VALUES ('john_doe'); tries to insert 'john_doe' which already exists, causing a UNIQUE violation.

3. Final Answer:

   INSERT INTO Users (username) VALUES ('john_doe'); -> Option C

4. Quick Check:

   Duplicate username violates UNIQUE constraint [OK]

Hint: UNIQUE means no duplicates allowed in that column [OK]

Common Mistakes:

• Assuming UPDATE causes error without duplicate
• Thinking inserts of new usernames cause error
• Confusing UNIQUE with PRIMARY KEY

5. A database table has two columns: OrderID and CustomerID. You want to ensure that each order belongs to an existing customer and that no two orders have the same OrderID. Which combination of integrity constraints should you use?

hard

A. NOT NULL on both columns only

B. UNIQUE on CustomerID and NOT NULL on OrderID

C. CHECK constraint on OrderID and UNIQUE on CustomerID

D. PRIMARY KEY on OrderID and FOREIGN KEY on CustomerID referencing Customers table

Solution

1. Step 1: Ensure unique OrderID

   Use PRIMARY KEY on OrderID to guarantee uniqueness and not null.

2. Step 2: Ensure Order belongs to existing Customer

   Use FOREIGN KEY on CustomerID referencing the Customers table to enforce valid customer links.

3. Final Answer:

   PRIMARY KEY on OrderID and FOREIGN KEY on CustomerID referencing Customers table -> Option D

4. Quick Check:

   Unique orders + valid customers = PRIMARY + FOREIGN KEY [OK]

Hint: Use PRIMARY KEY for unique ID, FOREIGN KEY for valid references [OK]

Common Mistakes:

• Using UNIQUE instead of PRIMARY KEY for OrderID
• Missing FOREIGN KEY for CustomerID
• Assuming NOT NULL alone enforces relationships