Composite index and column order in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Effect of column order in composite index on query performance

Consider a table Orders with columns customer_id and order_date. A composite index is created as (customer_id, order_date). Which query will benefit most from this index?

A SELECT * FROM Orders WHERE order_date = '2024-01-01';

B SELECT * FROM Orders WHERE order_date = '2024-01-01' AND customer_id = 123;

C SELECT * FROM Orders WHERE customer_id = 123 AND order_date = '2024-01-01';

D SELECT * FROM Orders WHERE customer_id = 123;

💡 Hint

Think about how the index is structured and which columns are used first in the WHERE clause.

The composite index (customer_id, order_date) is ordered first by customer_id, then by order_date. Queries filtering on customer_id and optionally order_date can use the index efficiently. Queries B and C both filter on both columns, so they benefit most. The order of conditions in the WHERE clause does not affect index usage. Query A filters only on order_date, which is the second column, so the index is not used efficiently. Query D filters only on customer_id, which can use the index but less specifically than B and C.

🧠 Conceptual

intermediate

Understanding composite index column order

Why does the order of columns in a composite index matter?

A Because the database uses the first column to quickly narrow down rows before checking the second column.

B Because the database always searches columns in alphabetical order regardless of index order.

C Because the order determines which columns can be NULL in the index.

D Because the order affects the physical storage location of the table rows.

💡 Hint

Think about how indexes help the database find data quickly.

The order of columns in a composite index matters because the database uses the first column to filter rows first, then uses the second column to further narrow down results. This makes searching efficient. The other options are incorrect because alphabetical order is not used, NULL handling is unrelated to order, and physical storage of rows is not affected by index column order.

📝 Syntax

advanced

Correct syntax to create a composite index

Which of the following SQL statements correctly creates a composite index on columns last_name and first_name in a table Employees?

A CREATE INDEX idx_name ON Employees last_name, first_name;

B CREATE INDEX idx_name ON Employees (last_name, first_name);

C CREATE INDEX idx_name (last_name, first_name) ON Employees;

D CREATE INDEX idx_name ON Employees WHERE last_name, first_name;

💡 Hint

Remember the order of clauses in the CREATE INDEX statement.

The correct syntax is CREATE INDEX index_name ON table_name (column1, column2);. Option B follows this syntax. Options A, C, and D have syntax errors such as missing parentheses or incorrect clause order.

❓ optimization

advanced

Choosing column order for composite index to optimize queries

You have a table Sales with columns region, sales_rep, and sale_date. Most queries filter by sales_rep and then by region. Which composite index column order is best for performance?

A (region, sale_date)

B (region, sales_rep)

C (sales_rep, region)

D (sale_date, sales_rep)

💡 Hint

Think about which column is filtered first in queries.

Since queries filter first by sales_rep and then by region, the composite index should start with sales_rep to efficiently narrow down rows, followed by region. Option C matches this order. Other options start with columns filtered later or not filtered at all, reducing index effectiveness.

🔧 Debug

expert

Diagnose why a composite index is not used

A composite index exists on (category, price) in a Products table. The query SELECT * FROM Products WHERE price > 100; is slow and does not use the index. Why?

A Because the query filters only on the second column of the composite index, the index cannot be used efficiently.

B Because the index must be rebuilt before it can be used for queries.

C Because the query uses a greater than operator which is not supported by indexes.

D Because the table has no primary key defined.

💡 Hint

Think about how composite indexes work with leading columns.

Composite indexes work best when the query filters on the first column or a leftmost prefix of the index. Since the query filters only on price, which is the second column, the index on (category, price) cannot be used efficiently. The other options are incorrect because indexes support range queries, rebuilding is not required for usage, and primary key presence does not affect index usage here.

Practice

1. What is the main benefit of using a composite index in a database?

easy

A. It encrypts the data for security.

B. It reduces the size of the database file.

C. It automatically backs up the database.

D. It speeds up queries filtering on multiple columns together.

Solution

1. Step 1: Understand what a composite index does

   A composite index is created on multiple columns to help the database find rows faster when queries filter on those columns together.

2. Step 2: Compare options to the definition

   Only It speeds up queries filtering on multiple columns together. correctly describes the benefit of speeding up queries using multiple columns. Other options describe unrelated features.

3. Final Answer:

   It speeds up queries filtering on multiple columns together. -> Option D

4. Quick Check:

   Composite index = speeds up multi-column queries [OK]

Hint: Composite index helps multi-column filters run faster [OK]

Common Mistakes:

• Thinking it reduces database size
• Confusing indexing with backup or encryption
• Assuming it speeds up single-column queries only

2. Which of the following is the correct syntax to create a composite index on columns last_name and first_name in a table employees?

easy

A. CREATE INDEX idx_name ON employees last_name, first_name;

B. CREATE INDEX idx_name ON employees (last_name, first_name);

C. CREATE INDEX idx_name (last_name, first_name) ON employees;

D. CREATE INDEX idx_name ON employees WHERE last_name, first_name;

Solution

1. Step 1: Recall correct CREATE INDEX syntax

   The correct syntax is: CREATE INDEX index_name ON table_name (column1, column2);

2. Step 2: Match options to syntax

   CREATE INDEX idx_name ON employees (last_name, first_name); matches the correct syntax exactly. Options A, C, and D have syntax errors or misplaced keywords.

3. Final Answer:

   CREATE INDEX idx_name ON employees (last_name, first_name); -> Option B

4. Quick Check:

   CREATE INDEX ... ON table (columns) [OK]

Hint: Use parentheses around columns after ON table [OK]

Common Mistakes:

• Omitting parentheses around columns
• Placing columns before ON keyword
• Using WHERE instead of column list

3. Given a composite index on (city, age) in a table users, which query will use the index efficiently?

medium

A. SELECT * FROM users WHERE age = 25 AND city = 'Boston';

B. SELECT * FROM users WHERE age > 30;

C. SELECT * FROM users WHERE city = 'Boston' AND age > 30;

D. SELECT * FROM users WHERE city = 'Boston' ORDER BY age DESC;

Solution

1. Step 1: Understand index column order usage

   A composite index on (city, age) can be used efficiently if the query filters on city first, then age.

2. Step 2: Analyze each query

   SELECT * FROM users WHERE city = 'Boston' AND age > 30; filters on city and age, matching index order. SELECT * FROM users WHERE age > 30; filters only on age (second column), so index is less useful. SELECT * FROM users WHERE age = 25 AND city = 'Boston'; filters age first, then city, which is less efficient. SELECT * FROM users WHERE city = 'Boston' ORDER BY age DESC; filters city but only orders by age, which may use index partially.

3. Final Answer:

   SELECT * FROM users WHERE city = 'Boston' AND age > 30; -> Option C

4. Quick Check:

   Index (city, age) best for city then age filters [OK]

Hint: Filter on first index column before others [OK]

Common Mistakes:

• Assuming index works well if filtering only second column
• Ignoring column order in composite index
• Thinking ORDER BY always uses index fully

4. You created a composite index on (department, salary) but your query filtering only on salary is slow. What is the likely cause?

medium

A. The index column order starts with department, so filtering only salary can't use it well.

B. The index is corrupted and needs rebuilding.

C. The salary column data type is incompatible with indexing.

D. The database does not support composite indexes.

Solution

1. Step 1: Recall how composite index column order affects usage

   Composite indexes work best when filtering starts with the first column. Filtering only on the second column (salary) won't use the index efficiently.

2. Step 2: Evaluate options

   The index column order starts with department, so filtering only salary can't use it well. correctly explains the cause. Options A, B, and D are unlikely or incorrect because the index exists and salary can be indexed normally.

3. Final Answer:

   The index column order starts with department, so filtering only salary can't use it well. -> Option A

4. Quick Check:

   Filter must start with first index column [OK]

Hint: Filter on first index column to use composite index [OK]

Common Mistakes:

• Assuming any column in index can be filtered efficiently alone
• Blaming index corruption without checking usage
• Ignoring column order importance

5. You have a table orders with columns customer_id, order_date, and status. You often run queries filtering by customer_id and sorting by order_date. Which composite index will optimize these queries best?

hard

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

B. CREATE INDEX idx_date_customer ON orders (order_date, customer_id);

C. CREATE INDEX idx_status_customer ON orders (status, customer_id);

D. CREATE INDEX idx_order ON orders (order_date);

Solution

1. Step 1: Identify query filter and sort columns

   Queries filter by customer_id and sort by order_date, so index should start with customer_id and include order_date next.

2. Step 2: Match index column order to query pattern

   CREATE INDEX idx_customer_date ON orders (customer_id, order_date); matches this order (customer_id, order_date), allowing efficient filtering and sorting. CREATE INDEX idx_date_customer ON orders (order_date, customer_id); reverses order, less efficient for filtering. Options A, B, and C do not match query needs.

3. Final Answer:

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

4. Quick Check:

   Index columns match filter then sort order [OK]

Hint: Index columns in filter then sort order [OK]

Common Mistakes:

• Reversing column order in composite index
• Indexing unrelated columns
• Using single-column index when composite needed