Column definitions and constraints in MySQL

Introduction

Columns store data in tables. Constraints make sure the data is correct and follows rules.

When creating a new table to define what kind of data each column holds.

When you want to make sure a column always has a value (not empty).

When you want to make sure values in a column are unique.

When you want to link data between tables using keys.

When you want to limit the type or size of data stored in a column.

Syntax

MySQL

column_name data_type [constraint1] [constraint2] ...

Data types define what kind of data the column holds, like INT for numbers or VARCHAR for text.

Constraints are rules like NOT NULL (no empty values), UNIQUE (no duplicates), PRIMARY KEY (unique ID), FOREIGN KEY (link to another table), and DEFAULT (default value).

Examples

This defines a column named id that holds integers and is the primary key (unique identifier).

MySQL

id INT PRIMARY KEY

This defines a name column that holds text up to 50 characters and cannot be empty.

MySQL

name VARCHAR(50) NOT NULL

This defines an email column that holds text and must be unique (no duplicates).

MySQL

email VARCHAR(100) UNIQUE

This defines an age column that holds numbers and defaults to 18 if no value is given.

MySQL

age INT DEFAULT 18

Sample Program

This creates a table named Users with four columns. user_id is the unique ID, username and email must be unique and not empty, and age defaults to 18 if not provided.

MySQL

CREATE TABLE Users (
  user_id INT PRIMARY KEY,
  username VARCHAR(30) NOT NULL UNIQUE,
  email VARCHAR(50) NOT NULL UNIQUE,
  age INT DEFAULT 18
);

Important Notes

Always choose the right data type to save space and improve speed.

Use NOT NULL to avoid missing data.

Primary keys uniquely identify each row and help link tables.

Summary

Columns define the kind of data stored in a table.

Constraints enforce rules to keep data accurate and consistent.

Common constraints include PRIMARY KEY, NOT NULL, UNIQUE, and DEFAULT.

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