Creating indexes in MySQL - Quick Revision & Summary

Recall & Review

beginner

What is an index in a database?

An index is like a shortcut in a book. It helps the database find data faster without looking at every row.

beginner

How do you create a simple index on a column named 'name' in MySQL?

Use the command: CREATE INDEX idx_name ON table_name(name); This makes searching by 'name' faster.

intermediate

What is the difference between a UNIQUE index and a regular index?

A UNIQUE index makes sure no two rows have the same value in the indexed column, while a regular index just speeds up searches.

intermediate

Why should you not create too many indexes on a table?

Too many indexes slow down data changes like INSERT, UPDATE, DELETE because the database must update all indexes too.

intermediate

What is a composite index?

A composite index is an index on multiple columns together. It helps speed up searches that use those columns combined.

Which command creates an index on the 'email' column in MySQL?

A CREATE INDEX idx_email ON users(email);

B CREATE TABLE idx_email ON users(email);

C INSERT INDEX idx_email ON users(email);

D ALTER TABLE users ADD COLUMN idx_email(email);

The correct syntax to create an index is CREATE INDEX index_name ON table_name(column_name);

What does a UNIQUE index ensure?

A No duplicate values in the indexed column

B Faster data insertion

C Allows NULL values only

D Deletes duplicate rows automatically

A UNIQUE index prevents duplicate values in the indexed column.

Which is a downside of having many indexes on a table?

A Indexes automatically fix data errors

B Slower data retrieval

C More disk space saved

D Slower data modification operations

More indexes mean the database must update them on data changes, slowing down INSERT, UPDATE, DELETE.

What is a composite index?

A An index on a single column

B An index on multiple columns together

C An index that stores data in compressed form

D An index that automatically updates

A composite index covers multiple columns combined for faster searches using those columns.

Which of these is NOT a valid reason to create an index?

A Speed up search queries

B Enforce uniqueness of data

C Make data insertion faster

D Improve join performance

Indexes slow down data insertion because they need updating; they do not make it faster.

Explain what an index is and why it is useful in a database.

Think about how you find a topic quickly in a book.

Describe how to create a simple index in MySQL and mention one important consideration when using indexes.

Remember the command and why too many indexes can be bad.

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