Query optimization strategies in DBMS Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the best index for faster query execution.

DBMS Theory

SELECT * FROM employees WHERE salary > 50000 ORDER BY [1];

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

Adepartment

Bname

Csalary

Dhire_date

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a column not used in the WHERE or ORDER BY clause.

Ignoring the column that filters the data.

The query orders results by salary, so using an index on the salary column helps speed up sorting and filtering.

2fill in blank

medium

Complete the code to rewrite the query using a join instead of a subquery for better performance.

DBMS Theory

SELECT e.name, d.department_name FROM employees e [1] departments d ON e.department_id = d.id;

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

AINNER JOIN

BLEFT JOIN

CRIGHT JOIN

DFULL JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using LEFT JOIN when only matching rows are needed.

Choosing a join type that returns extra rows unnecessarily.

Using an INNER JOIN replaces the subquery and returns only matching rows, which is often faster.

3fill in blank

hard

Fix the error in the query by choosing the correct clause to limit the number of rows returned.

DBMS Theory

SELECT * FROM sales [1] 10;

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ALIMIT

BORDER BY

CGROUP BY

DWHERE

Attempts:

3 left

💡 Hint

Common Mistakes

Using WHERE to limit rows instead of filtering conditions.

Confusing ORDER BY with limiting rows.

The LIMIT clause restricts the number of rows returned by the query, which helps optimize performance when only a subset is needed.

4fill in blank

hard

Fill both blanks to create a query that uses an index and filters rows efficiently.

DBMS Theory

SELECT * FROM orders WHERE [1] = 'shipped' AND [2] > '2023-01-01';

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

Astatus

Border_date

Ccustomer_id

Dtotal_amount

Attempts:

3 left

💡 Hint

Common Mistakes

Using columns unrelated to filtering conditions.

Choosing columns that are not indexed.

Filtering by status and order_date helps the database use indexes on these columns to quickly find relevant rows.

5fill in blank

hard

Fill all three blanks to write a query that aggregates data efficiently using grouping and filtering.

DBMS Theory

SELECT [1], COUNT(*) AS total FROM sales WHERE [2] > 100 GROUP BY [3];

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

Aregion

Bamount

Ddate

Attempts:

3 left

💡 Hint

Common Mistakes

Using different columns in SELECT and GROUP BY.

Filtering on a column not related to the aggregation.

The query groups sales by region, counts the number of sales where the amount is greater than 100, and uses the same column region in SELECT and GROUP BY for correct aggregation.

Practice

1. What is the main goal of query optimization in a database?

easy

A. To make data retrieval faster and more efficient

B. To increase the size of the database

C. To delete unnecessary data automatically

D. To encrypt data for security

Solution

1. Step 1: Understand the purpose of query optimization

   Query optimization aims to improve how quickly and efficiently data is retrieved from a database.

2. Step 2: Compare options with the goal

   Only To make data retrieval faster and more efficient matches this goal; others describe unrelated tasks.

3. Final Answer:

   To make data retrieval faster and more efficient -> Option A

4. Quick Check:

   Query optimization = faster data retrieval [OK]

Hint: Focus on speed and efficiency of data retrieval [OK]

Common Mistakes:

• Confusing optimization with data deletion
• Thinking optimization increases database size
• Mixing security tasks with optimization

2. Which of the following is a correct SQL syntax to create an index on the column employee_id in the table employees?

easy

A. CREATE employees INDEX idx_emp_id(employee_id);

B. MAKE INDEX idx_emp_id IN employees(employee_id);

C. INDEX CREATE idx_emp_id FOR employees(employee_id);

D. CREATE INDEX idx_emp_id ON employees(employee_id);

Solution

1. Step 1: Recall correct SQL syntax for creating an index

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

2. Step 2: Match options with correct syntax

   Only CREATE INDEX idx_emp_id ON employees(employee_id); matches the correct syntax exactly.

3. Final Answer:

   CREATE INDEX idx_emp_id ON employees(employee_id); -> Option D

4. Quick Check:

   CREATE INDEX ... ON ... (column) [OK]

Hint: Remember: CREATE INDEX index_name ON table(column) [OK]

Common Mistakes:

• Using wrong keywords like MAKE or FOR
• Placing table name before INDEX keyword
• Incorrect order of clauses

3. Consider the SQL query:
SELECT * FROM orders WHERE customer_id = 123;
If there is an index on customer_id, what is the expected effect on query performance?

medium

A. The query will return incorrect results

B. The query will run slower because indexes add overhead

C. The query will run faster by quickly locating matching rows

D. The query will ignore the index and scan the whole table

Solution

1. Step 1: Understand the role of indexes in queries

   Indexes help the database find rows matching conditions faster without scanning the entire table.

2. Step 2: Analyze the effect of an index on customer_id

   Since the query filters by customer_id, the index speeds up locating those rows.

3. Final Answer:

   The query will run faster by quickly locating matching rows -> Option C

4. Quick Check:

   Index on filter column = faster query [OK]

Hint: Index on filter column speeds up data retrieval [OK]

Common Mistakes:

• Thinking indexes slow down SELECT queries
• Assuming indexes cause wrong results
• Believing indexes are always ignored

4. A developer wrote this SQL query:
SELECT * FROM products WHERE price > 100 AND price < 50;
What is the main issue affecting query optimization here?

medium

A. The SELECT * syntax is invalid

B. The WHERE clause has conflicting conditions making the query return no rows

C. The query is missing an index on the price column

D. The table name is misspelled

Solution

1. Step 1: Analyze the WHERE clause conditions

   The conditions price > 100 and price < 50 cannot be true at the same time.

2. Step 2: Understand impact on query results and optimization

   This conflict means no rows will match, so the query returns empty results, wasting resources.

3. Final Answer:

   The WHERE clause has conflicting conditions making the query return no rows -> Option B

4. Quick Check:

   Conflicting WHERE conditions = no results [OK]

Hint: Check WHERE conditions for logical conflicts [OK]

Common Mistakes:

• Assuming SELECT * is wrong syntax
• Ignoring logical conflicts in conditions
• Thinking missing index causes no results

5. You have a large table sales with columns sale_date, region, and amount. You want to optimize this query:
SELECT region, SUM(amount) FROM sales WHERE sale_date BETWEEN '2023-01-01' AND '2023-01-31' GROUP BY region;
Which strategy will best improve performance?

hard

A. Create a composite index on (sale_date, region)

B. Create an index only on amount

C. Remove the GROUP BY clause

D. Use SELECT * instead of specific columns

Solution

1. Step 1: Identify columns used in WHERE and GROUP BY clauses

   The query filters by sale_date and groups by region.

2. Step 2: Choose an index that supports both filtering and grouping

   A composite index on (sale_date, region) helps quickly find rows in the date range and group them efficiently.

3. Final Answer:

   Create a composite index on (sale_date, region) -> Option A

4. Quick Check:

   Composite index on filter and group columns = better performance [OK]

Hint: Index columns used in WHERE and GROUP BY together [OK]

Common Mistakes:

• Indexing only the aggregated column
• Removing GROUP BY loses needed grouping
• Using SELECT * increases data load unnecessarily