Column definitions and constraints in MySQL - Mini Project: Build & Apply

Create a MySQL Table with Column Definitions and Constraints

📖 Scenario: You are setting up a simple database table to store information about books in a library. Each book has a unique ID, a title, an author, a publication year, and a price.

🎯 Goal: Create a MySQL table named books with columns that have specific data types and constraints to ensure data is stored correctly.

📋 What You'll Learn

Create a table named books

Add a column book_id as an integer and set it as the primary key

Add a column title as a variable character string with a maximum length of 100 characters and not allow NULL values

Add a column author as a variable character string with a maximum length of 50 characters and not allow NULL values

Add a column pub_year as an integer with a constraint that it cannot be less than 1500

Add a column price as a decimal number with two decimal places and a default value of 0.00

💡 Why This Matters

🌍 Real World

Defining tables with proper column types and constraints is essential for storing accurate and reliable data in any database system.

💼 Career

Database administrators and developers must know how to define tables with constraints to maintain data quality and support application requirements.

1

Create the table structure

Write the SQL statement to create a table named books with columns book_id as INT, title as VARCHAR(100), author as VARCHAR(50), pub_year as INT, and price as DECIMAL(5,2).

MySQL

CREATE TABLE books (
    book_id INT,
    title VARCHAR(100),
    author VARCHAR(50),
    pub_year INT,
    price DECIMAL(5,2)
);
-- Your code here

Hint

Start by defining the table and columns with their data types.

2

Add primary key and NOT NULL constraints

Modify the books table creation to set book_id as the primary key and add NOT NULL constraints to title and author columns.

MySQL

CREATE TABLE books (
    book_id INT,
    title VARCHAR(100),
    author VARCHAR(50),
    pub_year INT,
    price DECIMAL(5,2),
    -- Your code here to add primary key and NOT NULL constraints
);

Hint

Use PRIMARY KEY after book_id INT and add NOT NULL after title VARCHAR(100) and author VARCHAR(50).

3

Add a CHECK constraint for publication year

Add a CHECK constraint to the pub_year column so that the year cannot be less than 1500.

MySQL

CREATE TABLE books (
    book_id INT PRIMARY KEY,
    title VARCHAR(100) NOT NULL,
    author VARCHAR(50) NOT NULL,
    pub_year INT CHECK (pub_year >= 1500),
    price DECIMAL(5,2)
);
-- Your code here to add CHECK constraint

Hint

Use CHECK (pub_year >= 1500) after the pub_year INT column definition.

4

Set default value for price column

Add a DEFAULT value of 0.00 to the price column in the books table.

MySQL

CREATE TABLE books (
    book_id INT PRIMARY KEY,
    title VARCHAR(100) NOT NULL,
    author VARCHAR(50) NOT NULL,
    pub_year INT CHECK (pub_year >= 1500),
    price DECIMAL(5,2) DEFAULT 0.00
);
-- Your code here to add DEFAULT value

Hint

Add DEFAULT 0.00 after the price DECIMAL(5,2) column definition.

Practice

1. Which constraint ensures that a column cannot have empty (NULL) values in MySQL?

easy

A. NOT NULL

B. UNIQUE

C. PRIMARY KEY

D. DEFAULT

Solution

1. Step 1: Understand the purpose of NOT NULL

   The NOT NULL constraint prevents a column from having NULL values, meaning it must always have data.

2. Step 2: Compare with other constraints

   UNIQUE allows NULLs but no duplicates, PRIMARY KEY implies NOT NULL and uniqueness, and DEFAULT sets a default value but does not prevent NULL.

3. Final Answer:

   NOT NULL -> Option A

4. Quick Check:

   Prevent empty values = NOT NULL [OK]

Hint: NOT NULL means no empty values allowed [OK]

Common Mistakes:

• Confusing UNIQUE with NOT NULL
• Thinking DEFAULT prevents NULL
• Assuming PRIMARY KEY is only for uniqueness

2. Which of the following is the correct way to define a column named id as an auto-incrementing primary key in MySQL?

easy

A. id AUTO_INCREMENT INT PRIMARY KEY

B. id INT AUTO_INCREMENT PRIMARY

C. id INT PRIMARY KEY AUTO_INCREMENT

D. id PRIMARY KEY INT AUTO_INCREMENT

Solution

1. Step 1: Recall correct column definition order

   In MySQL, the correct syntax places the data type first, then constraints like PRIMARY KEY and AUTO_INCREMENT.

2. Step 2: Validate each option's syntax

   id INT PRIMARY KEY AUTO_INCREMENT follows the correct order: id INT PRIMARY KEY AUTO_INCREMENT. Others have incorrect order or missing keywords.

3. Final Answer:

   id INT PRIMARY KEY AUTO_INCREMENT -> Option C

4. Quick Check:

   Data type first, then constraints [OK]

Hint: Data type first, then PRIMARY KEY and AUTO_INCREMENT [OK]

Common Mistakes:

• Placing AUTO_INCREMENT before data type
• Using PRIMARY instead of PRIMARY KEY
• Mixing order of keywords

3. Given the table definition:

CREATE TABLE users (
  user_id INT PRIMARY KEY,
  email VARCHAR(100) UNIQUE NOT NULL,
  age INT DEFAULT 18
);

What will happen if you try to insert:

INSERT INTO users (user_id, email) VALUES (1, NULL);

medium

A. Insert fails due to NOT NULL constraint on email

B. Insert succeeds with email set to NULL

C. Insert succeeds with email set to default value

D. Insert fails due to UNIQUE constraint violation

Solution

1. Step 1: Check the email column constraints

   Email is defined as UNIQUE NOT NULL, so it cannot be NULL.

2. Step 2: Analyze the insert statement

   The insert tries to set email to NULL, violating the NOT NULL constraint, causing an error.

3. Final Answer:

   Insert fails due to NOT NULL constraint on email -> Option A

4. Quick Check:

   NOT NULL rejects NULL values [OK]

Hint: NOT NULL columns reject NULL inserts [OK]

Common Mistakes:

• Assuming DEFAULT applies when value is NULL
• Confusing UNIQUE with NOT NULL
• Thinking NULL is allowed if UNIQUE

4. Consider this table creation statement:

CREATE TABLE products (
  product_id INT PRIMARY KEY,
  name VARCHAR(50) UNIQUE,
  price DECIMAL(5,2) NOT NULL DEFAULT 0.00
);

Which of the following statements will cause an error?

medium

A. INSERT INTO products (product_id, name, price) VALUES (1, 'Pen', 1.50);

B. INSERT INTO products (product_id, name) VALUES (2, 'Book');

C. INSERT INTO products (product_id, price) VALUES (3, 2.00);

D. INSERT INTO products (product_id, name, price) VALUES (1, 'Pencil', 0.50);

Solution

1. Step 1: Check constraints on each insert

   INSERT INTO products (product_id, name, price) VALUES (1, 'Pen', 1.50); inserts all required fields correctly. INSERT INTO products (product_id, name) VALUES (2, 'Book'); omits price but uses default 0.00, allowed. INSERT INTO products (product_id, price) VALUES (3, 2.00); omits name, which is nullable, allowed.

2. Step 2: Identify duplicate primary key error

   INSERT INTO products (product_id, name, price) VALUES (1, 'Pencil', 0.50); tries to insert product_id 1 again, violating the PRIMARY KEY uniqueness constraint, causing an error.

3. Final Answer:

   INSERT INTO products (product_id, name, price) VALUES (1, 'Pencil', 0.50); -> Option D

4. Quick Check:

   PRIMARY KEY must be unique [OK]

Hint: Duplicate PRIMARY KEY values cause errors [OK]

Common Mistakes:

• Assuming UNIQUE allows duplicates
• Forgetting DEFAULT applies only if column omitted
• Ignoring PRIMARY KEY uniqueness

5. You want to create a table orders where order_id is a primary key, customer_email must be unique and not null, and order_date defaults to the current date if not provided. Which of the following column definitions is correct?

hard

A. order_id INT PRIMARY KEY, customer_email VARCHAR(100) NOT NULL UNIQUE, order_date DATE DEFAULT NOW()

B. order_id INT PRIMARY KEY, customer_email VARCHAR(100) UNIQUE NOT NULL, order_date DATE DEFAULT CURRENT_DATE

C. order_id INT PRIMARY KEY, customer_email VARCHAR(100) UNIQUE, order_date DATE DEFAULT CURRENT_TIMESTAMP

D. order_id INT PRIMARY KEY, customer_email VARCHAR(100) NOT NULL UNIQUE, order_date DATE DEFAULT CURDATE()

Solution

1. Step 1: Check constraints on customer_email

   The column must be both UNIQUE and NOT NULL. Using either UNIQUE NOT NULL or NOT NULL UNIQUE is valid in MySQL.

2. Step 2: Verify default for order_date

   MySQL supports DEFAULT CURRENT_DATE for DATE columns. NOW() and CURRENT_TIMESTAMP return datetime, not date, causing errors. CURDATE() is a function and cannot be used directly as default.

3. Step 3: Confirm full correctness

   order_id INT PRIMARY KEY, customer_email VARCHAR(100) UNIQUE NOT NULL, order_date DATE DEFAULT CURRENT_DATE uses correct syntax for all constraints and default value.

4. Final Answer:

   order_id INT PRIMARY KEY, customer_email VARCHAR(100) UNIQUE NOT NULL, order_date DATE DEFAULT CURRENT_DATE -> Option B

5. Quick Check:

   Use CURRENT_DATE for DATE default [OK]

Hint: Use CURRENT_DATE for DATE default, UNIQUE NOT NULL order matters [OK]

Common Mistakes:

• Using NOW() or CURRENT_TIMESTAMP for DATE default
• Trying to use functions like CURDATE() as default
• Confusing UNIQUE and NOT NULL order