Integrity constraints in DBMS Theory - Mini Project: Build & Apply

Understanding Integrity Constraints in Databases

📖 Scenario: You are designing a simple database for a small library. To keep the data accurate and reliable, you need to apply rules called integrity constraints.These rules help ensure that the data entered into the database follows certain conditions, like no duplicate book IDs or valid publication years.

🎯 Goal: Build a basic understanding of integrity constraints by creating a table with specific rules that keep the library data correct and consistent.

📋 What You'll Learn

Create a table named Books with columns BookID, Title, Author, and YearPublished

Add a primary key constraint on BookID to prevent duplicate entries

Add a check constraint on YearPublished to allow only years from 1500 to the current year

Add a not null constraint on Title and Author to ensure these fields are always filled

💡 Why This Matters

🌍 Real World

Libraries, schools, and businesses use integrity constraints to keep their data trustworthy and consistent.

💼 Career

Database administrators and developers use integrity constraints to design robust databases that prevent errors and maintain data quality.

1

Create the Books table with columns

Write a SQL statement to create a table called Books with columns BookID as integer, Title as text, Author as text, and YearPublished as integer.

DBMS Theory

CREATE TABLE Books (
    BookID INTEGER,
    Title TEXT,
    Author TEXT,
    YearPublished INTEGER
);
-- Your code here

Hint

Use CREATE TABLE followed by the table name and define each column with its data type inside parentheses.

2

Add primary key and not null constraints

Modify the Books table creation to add a primary key constraint on BookID and add NOT NULL constraints on Title and Author columns.

DBMS Theory

CREATE TABLE Books (
    BookID INTEGER PRIMARY KEY,
    Title TEXT NOT NULL,
    Author TEXT NOT NULL,
    YearPublished INTEGER
);
-- Your code here

Hint

Add PRIMARY KEY after BookID INTEGER and NOT NULL after Title TEXT and Author TEXT.

3

Add a check constraint on YearPublished

Add a CHECK constraint to the YearPublished column to allow only years between 1500 and 2024 inclusive.

DBMS Theory

CREATE TABLE Books (
    BookID INTEGER PRIMARY KEY,
    Title TEXT NOT NULL,
    Author TEXT NOT NULL,
    YearPublished INTEGER CHECK (YearPublished >= 1500 AND YearPublished <= 2024)
);
-- Your code here

Hint

Use CHECK (YearPublished >= 1500 AND YearPublished <= 2024) after the YearPublished INTEGER column definition.

4

Complete the Books table with all constraints

Ensure the Books table creation includes the primary key on BookID, NOT NULL on Title and Author, and the CHECK constraint on YearPublished from 1500 to 2024.

DBMS Theory

CREATE TABLE Books (
    BookID INTEGER PRIMARY KEY,
    Title TEXT NOT NULL,
    Author TEXT NOT NULL,
    YearPublished INTEGER CHECK (YearPublished >= 1500 AND YearPublished <= 2024)
);
-- Your code here

Hint

Review all constraints together to make sure the table enforces data integrity.

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