Composite index and column order in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a composite index on columns 'last_name' and 'first_name'.

SQL

CREATE INDEX idx_name ON employees([1], first_name);

Drag options to blanks, or click blank then click option'

Alast_name

Bemployee_id

Cfirst_name

Ddepartment

Attempts:

3 left

💡 Hint

Common Mistakes

Putting 'first_name' before 'last_name' which may reduce index efficiency.

Using a column not relevant to the query.

The composite index should start with 'last_name' followed by 'first_name' to optimize queries filtering by last name first.

2fill in blank

medium

Complete the code to select rows using the composite index on 'last_name' and 'first_name'.

SQL

SELECT * FROM employees WHERE last_name = 'Smith' AND [1] = 'John';

Drag options to blanks, or click blank then click option'

Adepartment

Bfirst_name

Cemployee_id

Dhire_date

Attempts:

3 left

💡 Hint

Common Mistakes

Filtering on columns not part of the composite index.

Using only one column in the WHERE clause when both are needed.

The composite index is on 'last_name' and 'first_name', so the query should filter on both columns to use the index efficiently.

3fill in blank

hard

Fix the error in the composite index creation by placing columns in the correct order.

SQL

CREATE INDEX idx_order ON orders([1], order_date);

Drag options to blanks, or click blank then click option'

Acustomer_id

Border_date

Cproduct_id

Dstatus

Attempts:

3 left

💡 Hint

Common Mistakes

Putting 'order_date' before 'customer_id' which reduces index effectiveness.

Choosing unrelated columns for the index.

The composite index should start with 'customer_id' before 'order_date' to optimize queries filtering by customer first.

4fill in blank

hard

Fill both blanks to create a composite index on 'category' and 'price' columns.

SQL

CREATE INDEX idx_product ON products([1], [2]);

Drag options to blanks, or click blank then click option'

Acategory

Bprice

Cname

Dstock

Attempts:

3 left

💡 Hint

Common Mistakes

Reversing the order of columns in the index.

Using columns not related to filtering.

The composite index should be on 'category' first, then 'price' to optimize queries filtering by category and price range.

5fill in blank

hard

Fill all three blanks to create a composite index on 'region', 'sales_rep', and 'sale_date' columns.

SQL

CREATE INDEX idx_sales ON sales([1], [2], [3]);

Drag options to blanks, or click blank then click option'

Aregion

Bsales_rep

Csale_date

Dcustomer_id

Attempts:

3 left

💡 Hint

Common Mistakes

Changing the order of columns which affects index usage.

Including columns not used in filtering.

The composite index should be created with 'region' first, then 'sales_rep', and finally 'sale_date' to optimize queries filtering by these columns in this order.

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