Single column index in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a single column index in a database?

A single column index is a database structure that speeds up data retrieval by creating an index on just one column of a table.

beginner

How does a single column index improve query performance?

It allows the database to quickly locate rows based on the indexed column without scanning the entire table.

beginner

Which SQL statement creates a single column index on the column 'email' in the 'users' table?

CREATE INDEX idx_email ON users(email);

beginner

True or False: A single column index can be used to speed up queries filtering by that column only.

True. It is designed to optimize queries that filter or sort by the indexed column.

intermediate

What is a potential downside of creating too many single column indexes?

Too many indexes can slow down data insertion, updates, and deletions because the indexes need to be updated as well.

What does a single column index do?

A Speeds up queries filtering on one column

B Stores all table data in one column

C Deletes duplicate rows automatically

D Creates a backup of the database

A single column index helps the database find rows faster when filtering by that column.

Which SQL command creates a single column index on 'username' in table 'accounts'?

A DROP INDEX idx_username;

B CREATE TABLE idx_username (username);

C CREATE INDEX idx_username ON accounts(username);

D SELECT * FROM accounts WHERE username;

The CREATE INDEX statement creates an index on the specified column.

What happens if you create many single column indexes on a table?

A Table size decreases

B Inserts and updates become slower

C Queries become slower

D Database crashes

Indexes need updating on data changes, so many indexes slow down inserts and updates.

Can a single column index speed up queries filtering by multiple columns?

A No, it only helps with one column

B Yes, always

C Only if columns are combined in the index

D Only if the table is small

A single column index only helps queries filtering on that one column.

Which of these is a benefit of using a single column index?

A Compresses the table data

B Automatic data backup

C Prevents data duplication

D Faster data retrieval on that column

Indexes improve speed of data retrieval based on the indexed column.

Explain what a single column index is and how it helps database queries.

Think about how an index in a book helps you find a topic quickly.

Describe the tradeoffs of using single column indexes in a database.

Consider both benefits and costs of indexes.

Practice

1.

What is the main purpose of creating a single column index in a database?

easy

A. To store duplicate values in the column

B. To automatically backup the database

C. To prevent any changes to the column data

D. To speed up queries that filter or sort by that column

Solution

1. Step 1: Understand what an index does

   An index helps the database find rows faster when searching or sorting by the indexed column.

2. Step 2: Identify the purpose of a single column index

   A single column index speeds up queries that use that column in WHERE or ORDER BY clauses.

3. Final Answer:

   To speed up queries that filter or sort by that column -> Option D

4. Quick Check:

   Index speeds up queries = B [OK]

Hint: Index speeds up searches on one column [OK]

Common Mistakes:

• Thinking index stores duplicate values
• Believing index prevents data changes
• Confusing index with backup

2.

Which of the following is the correct syntax to create a single column index on the email column of a table named users?

easy

A. CREATE INDEX users ON idx_email(email);

B. CREATE INDEX idx_email ON users(email);

C. CREATE INDEX email ON users idx_email;

D. CREATE users INDEX idx_email(email);

Solution

1. Step 1: Recall the syntax for creating an index

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

2. Step 2: Match the syntax with the options

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

3. Final Answer:

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

4. Quick Check:

   CREATE INDEX index_name ON table(column) = A [OK]

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

Common Mistakes:

• Swapping table and index names
• Missing ON keyword
• Incorrect order of keywords

3.

Given a table products with columns id, name, and category, and a single column index on category, what will the query below most likely do?

SELECT * FROM products WHERE category = 'Books';

medium

A. Use the index to quickly find rows where category is 'Books'

B. Sort the results by category before returning

C. Return an error because index cannot be used in WHERE clause

D. Scan the entire table without using the index

Solution

1. Step 1: Understand how an index affects a WHERE query

   An index on the category column helps the database quickly locate rows matching the condition category = 'Books'.

2. Step 2: Analyze the query behavior with the index

   The query will use the index to avoid scanning the whole table, speeding up the search.

3. Final Answer:

   Use the index to quickly find rows where category is 'Books' -> Option A

4. Quick Check:

   Index speeds WHERE search = D [OK]

Hint: WHERE on indexed column uses index for speed [OK]

Common Mistakes:

• Assuming index is ignored in WHERE clause
• Thinking index causes errors in queries
• Confusing index use with sorting

4.

Consider this SQL statement to create an index:

CREATE INDEX idx_price ON products price;

What is wrong with this statement?

medium

A. Table name should come after the column name

B. Index name cannot start with 'idx_'

C. Missing parentheses around the column name

D. CREATE INDEX cannot be used on numeric columns

Solution

1. Step 1: Check the syntax for CREATE INDEX

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

2. Step 2: Identify the syntax error

   The statement is missing parentheses around price. It should be ON products(price).

3. Final Answer:

   Missing parentheses around the column name -> Option C

4. Quick Check:

   Column name needs parentheses = C [OK]

Hint: Column name must be in parentheses after table [OK]

Common Mistakes:

• Omitting parentheses around column
• Misplacing table and column names
• Thinking index names have restrictions

5.

You have a table orders with columns order_id, customer_id, and order_date. You want to speed up queries that filter by customer_id and sort by order_date. Which approach is best?

hard

A. Create a composite index on customer_id and order_date

B. Create a single column index on order_date only

C. Create a single column index on customer_id only

D. Create no index to avoid slowing down inserts

Solution

1. Step 1: Understand query needs

   Queries filter by customer_id and sort by order_date, so both columns matter.

2. Step 2: Choose the best index type

   A composite index on both columns helps filter and sort efficiently, better than single column indexes.

3. Final Answer:

   Create a composite index on customer_id and order_date -> Option A

4. Quick Check:

   Composite index for filter + sort = A [OK]

Hint: Use composite index for multiple column queries [OK]

Common Mistakes:

• Using single column index when composite is better
• Avoiding indexes fearing insert slowdown
• Indexing only sort column ignoring filter