Primary key declaration in MySQL

Introduction

A primary key helps to uniquely identify each row in a table so you can find or update data easily.

When creating a new table to ensure each record is unique.

When you want to quickly find a specific row in a large table.

When you want to prevent duplicate records in a table.

When linking tables together using relationships.

When updating or deleting a specific record safely.

Syntax

MySQL

CREATE TABLE table_name (
  column_name datatype PRIMARY KEY,
  other_column datatype
);

The PRIMARY KEY column must have unique values and cannot be NULL.

You can declare the primary key inline with the column or separately after all columns.

Examples

Primary key declared inline with the StudentID column.

MySQL

CREATE TABLE Students (
  StudentID INT PRIMARY KEY,
  Name VARCHAR(100)
);

Primary key declared separately after all columns.

MySQL

CREATE TABLE Employees (
  EmployeeID INT,
  Name VARCHAR(100),
  PRIMARY KEY (EmployeeID)
);

Table with only one column as primary key.

MySQL

CREATE TABLE EmptyTable (
  ID INT PRIMARY KEY
);

Sample Program

This creates a Books table with BookID as the primary key, inserts two books, and then selects all rows.

MySQL

CREATE TABLE Books (
  BookID INT PRIMARY KEY,
  Title VARCHAR(255),
  Author VARCHAR(255)
);

INSERT INTO Books (BookID, Title, Author) VALUES
(1, 'The Hobbit', 'J.R.R. Tolkien'),
(2, '1984', 'George Orwell');

SELECT * FROM Books;

Important Notes

Primary key columns are automatically indexed for faster searches.

Trying to insert duplicate or NULL values in a primary key column will cause an error.

Use primary keys to link tables with foreign keys for data integrity.

Summary

Primary keys uniquely identify each row in a table.

They prevent duplicate and NULL values in the key column.

You can declare primary keys inline or separately in the table definition.

Practice

1. What is the main purpose of a PRIMARY KEY in a MySQL table?

easy

A. To uniquely identify each row and prevent duplicates

B. To allow NULL values in a column

C. To speed up SELECT queries only

D. To store large text data

Solution

1. Step 1: Understand the role of PRIMARY KEY

   A PRIMARY KEY uniquely identifies each row in a table and ensures no duplicate or NULL values exist in that column.

2. Step 2: Compare options with PRIMARY KEY purpose

   Only To uniquely identify each row and prevent duplicates correctly states the unique identification and prevention of duplicates, which is the main purpose.

3. Final Answer:

   To uniquely identify each row and prevent duplicates -> Option A

4. Quick Check:

   Primary key = unique row ID [OK]

Hint: Primary key means unique and no duplicates [OK]

Common Mistakes:

• Thinking primary key allows NULL values
• Confusing primary key with indexing only
• Assuming primary key stores large text

2. Which of the following is the correct syntax to declare a primary key inline in a MySQL CREATE TABLE statement?

easy

A. CREATE TABLE users (id INT, name VARCHAR(50 PRIMARY KEY));

B. CREATE TABLE users (id INT, name VARCHAR(50), PRIMARY KEY id);

C. CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(50));

D. CREATE TABLE users (id INT, PRIMARY KEY, name VARCHAR(50));

Solution

1. Step 1: Review inline primary key syntax

   Inline primary key declaration is done by adding PRIMARY KEY right after the column type, like id INT PRIMARY KEY.

2. Step 2: Check each option

   CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(50)); correctly places PRIMARY KEY inline after id INT. Others have syntax errors or misplaced keywords.

3. Final Answer:

   CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(50)); -> Option C

4. Quick Check:

   Inline primary key = column type + PRIMARY KEY [OK]

Hint: Inline primary key goes right after column type [OK]

Common Mistakes:

• Placing PRIMARY KEY after column list without parentheses
• Using PRIMARY KEY without column name
• Putting PRIMARY KEY after another column

3. Given the table creation below, what will be the result of inserting two rows with the same id?

CREATE TABLE products (
  id INT PRIMARY KEY,
  name VARCHAR(100)
);
INSERT INTO products (id, name) VALUES (1, 'Pen');
INSERT INTO products (id, name) VALUES (1, 'Pencil');

medium

A. Both rows inserted successfully

B. Error due to duplicate primary key value

C. Second row overwrites the first row

D. NULL inserted for the second row's id

Solution

1. Step 1: Understand primary key uniqueness

   The primary key column id must have unique values. Duplicate values cause errors.

2. Step 2: Analyze the insert statements

   The first insert adds id=1. The second insert tries to add the same id=1, violating uniqueness.

3. Final Answer:

   Error due to duplicate primary key value -> Option B

4. Quick Check:

   Duplicate primary key = error [OK]

Hint: Primary key duplicates cause insert errors [OK]

Common Mistakes:

• Assuming duplicate inserts overwrite existing rows
• Thinking NULL can be inserted in primary key
• Believing both rows insert without error

4. Identify the error in the following MySQL table creation statement:

CREATE TABLE orders (
  order_id INT,
  customer_id INT,
  PRIMARY KEY order_id
);

medium

A. No error, statement is correct

B. Primary key cannot be declared separately

C. order_id must be VARCHAR type

D. Missing parentheses around the primary key column

Solution

1. Step 1: Check syntax for separate primary key declaration

   When declaring a primary key separately, the column name(s) must be inside parentheses, like PRIMARY KEY (order_id).

2. Step 2: Identify the error in the statement

   The statement uses PRIMARY KEY order_id without parentheses, causing a syntax error.

3. Final Answer:

   Missing parentheses around the primary key column -> Option D

4. Quick Check:

   Separate primary key needs parentheses [OK]

Hint: Separate primary key needs ( ) around columns [OK]

Common Mistakes:

• Omitting parentheses in separate primary key
• Thinking primary key must be inline only
• Assuming INT type is invalid for primary key

5. You want to create a table employees with a composite primary key consisting of department_id and employee_number. Which is the correct way to declare this primary key?

hard

A. CREATE TABLE employees (department_id INT, employee_number INT, PRIMARY KEY (department_id, employee_number));

B. CREATE TABLE employees (department_id INT PRIMARY KEY, employee_number INT PRIMARY KEY);

C. CREATE TABLE employees (department_id INT, employee_number INT PRIMARY KEY);

D. CREATE TABLE employees (department_id INT, employee_number INT, PRIMARY KEY department_id, employee_number);

Solution

1. Step 1: Understand composite primary key syntax

   A composite primary key is declared by listing multiple columns inside parentheses after PRIMARY KEY.

2. Step 2: Evaluate each option

   CREATE TABLE employees (department_id INT, employee_number INT, PRIMARY KEY (department_id, employee_number)); correctly declares both columns together as a composite primary key. CREATE TABLE employees (department_id INT PRIMARY KEY, employee_number INT PRIMARY KEY); tries to assign two primary keys separately, which is invalid. CREATE TABLE employees (department_id INT, employee_number INT PRIMARY KEY); declares only one column as primary key. CREATE TABLE employees (department_id INT, employee_number INT, PRIMARY KEY department_id, employee_number); has syntax errors missing parentheses.

3. Final Answer:

   CREATE TABLE employees (department_id INT, employee_number INT, PRIMARY KEY (department_id, employee_number)); -> Option A

4. Quick Check:

   Composite key = PRIMARY KEY (col1, col2) [OK]

Hint: Composite keys list columns inside parentheses [OK]

Common Mistakes:

• Declaring multiple primary keys separately
• Missing parentheses for composite keys
• Assigning primary key to only one column