Creating indexes in MySQL

Introduction

Indexes help the database find data faster, like a shortcut in a book's table of contents.

When you want to speed up searches on a large table.

When you often look up data by a specific column, like a username or email.

When you join tables on certain columns frequently.

When you want to enforce uniqueness on a column to avoid duplicates.

Syntax

MySQL

CREATE INDEX index_name ON table_name (column_name);

-- For unique values:
CREATE UNIQUE INDEX index_name ON table_name (column_name);

You can create an index on one or more columns by listing them inside the parentheses separated by commas.

Unique indexes ensure no two rows have the same value in the indexed column(s).

Examples

Creates an index named 'idx_lastname' on the 'last_name' column of the 'employees' table.

MySQL

CREATE INDEX idx_lastname ON employees (last_name);

Creates a unique index on the 'email' column to prevent duplicate emails in the 'users' table.

MySQL

CREATE UNIQUE INDEX idx_email ON users (email);

Creates an index on two columns, 'last_name' and 'date_of_birth', to speed up queries filtering by both.

MySQL

CREATE INDEX idx_name_dob ON persons (last_name, date_of_birth);

Sample Program

This creates a 'books' table and adds an index on the 'author' column. The EXPLAIN statement shows how MySQL uses the index to find rows faster.

MySQL

CREATE TABLE books (
  id INT PRIMARY KEY,
  title VARCHAR(100),
  author VARCHAR(50),
  published_year INT
);

CREATE INDEX idx_author ON books (author);

-- Query to test the index
EXPLAIN SELECT * FROM books WHERE author = 'Jane Austen';

Important Notes

Indexes speed up data retrieval but can slow down data insertion and updates because the index must be updated too.

Use indexes on columns that you search or join on often, but avoid adding too many indexes.

You can drop an index with: DROP INDEX index_name ON table_name;

Summary

Indexes make searching data faster by creating shortcuts.

Create indexes on columns you use often in searches or joins.

Unique indexes prevent duplicate values in a column.

Practice

1. What is the main purpose of creating an index on a column in a MySQL table?

easy

A. To speed up data retrieval when searching or filtering

B. To store duplicate values in the column

C. To increase the size of the database

D. To automatically backup the table data

Solution

1. Step 1: Understand what an index does

   An index acts like a shortcut to find data faster in a table.

2. Step 2: Identify the main benefit

   Indexes speed up searches and filtering by avoiding full table scans.

3. Final Answer:

   To speed up data retrieval when searching or filtering -> Option A

4. Quick Check:

   Index = faster search [OK]

Hint: Indexes speed up searches by creating shortcuts [OK]

Common Mistakes:

• Thinking indexes store duplicate values
• Believing indexes increase database size significantly
• Confusing indexes with backups

2. Which of the following is the correct syntax to create a simple index on the column email in a table named users?

easy

A. INDEX CREATE email_index ON users (email);

B. CREATE INDEX email_index ON users (email);

C. CREATE INDEX users.email_index (email);

D. CREATE INDEX ON users email_index (email);

Solution

1. Step 1: Recall MySQL index creation syntax

   The correct syntax is: CREATE INDEX index_name ON table_name (column_name);

2. Step 2: Match the syntax with options

   CREATE INDEX email_index ON users (email); matches the correct syntax exactly.

3. Final Answer:

   CREATE INDEX email_index ON users (email); -> Option B

4. Quick Check:

   CREATE INDEX index_name ON table (column) [OK]

Hint: Syntax: CREATE INDEX name ON table (column) [OK]

Common Mistakes:

• Swapping order of table and index name
• Omitting ON keyword
• Using wrong keyword order

3. Given the table products with columns id, name, and category, what will be the effect of this query?

CREATE UNIQUE INDEX idx_name ON products (name);

medium

A. It allows duplicate product names in the table

B. It creates a backup of the products table

C. It deletes rows with duplicate names

D. It prevents duplicate product names and speeds up searches by name

Solution

1. Step 1: Understand UNIQUE index effect

   A UNIQUE index prevents duplicate values in the indexed column.

2. Step 2: Identify additional benefit

   Indexes also speed up searches on that column.

3. Final Answer:

   It prevents duplicate product names and speeds up searches by name -> Option D

4. Quick Check:

   UNIQUE index = no duplicates + faster search [OK]

Hint: UNIQUE index blocks duplicates and speeds search [OK]

Common Mistakes:

• Thinking UNIQUE index allows duplicates
• Confusing index with backup or delete
• Ignoring speed improvement

4. You try to create an index with this command:

CREATE INDEX idx_age ON employees age;

What is the error in this statement?

medium

A. Table name is missing

B. Index name cannot start with 'idx_'

C. Missing parentheses around the column name

D. Cannot create index on numeric columns

Solution

1. Step 1: Check syntax for CREATE INDEX

   The column list must be inside parentheses after the table name.

2. Step 2: Identify the error

   The statement lacks parentheses around age, causing syntax error.

3. Final Answer:

   Missing parentheses around the column name -> Option C

4. Quick Check:

   Columns must be in parentheses [OK]

Hint: Always put columns in parentheses after table name [OK]

Common Mistakes:

• Forgetting parentheses around columns
• Thinking index names have restrictions
• Assuming numeric columns can't be indexed

5. You have a large orders table with columns order_id, customer_id, and order_date. You often query orders by customer_id and order_date. Which index creation will best improve query speed without allowing duplicate entries for the same customer on the same date?

hard

A. CREATE UNIQUE INDEX idx_customer_date ON orders (customer_id, order_date);

B. CREATE INDEX idx_customer ON orders (customer_id);

C. CREATE INDEX idx_order_date ON orders (order_date);

D. CREATE UNIQUE INDEX idx_order_id ON orders (order_id);

Solution

1. Step 1: Analyze query needs

   Queries filter by both customer_id and order_date, so a composite index on both is best.

2. Step 2: Consider uniqueness requirement

   To prevent duplicate orders for the same customer on the same date, use a UNIQUE index on both columns.

3. Final Answer:

   CREATE UNIQUE INDEX idx_customer_date ON orders (customer_id, order_date); -> Option A

4. Quick Check:

   Composite UNIQUE index matches query and uniqueness [OK]

Hint: Use composite UNIQUE index for multi-column uniqueness and speed [OK]

Common Mistakes:

• Creating single-column indexes only
• Ignoring uniqueness requirement
• Indexing unrelated columns