Subquery vs JOIN performance trade-off in SQL - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all customers who have placed an order using a subquery.

SQL

SELECT customer_id FROM orders WHERE customer_id IN (SELECT [1] FROM customers);

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

Acustomer_id

Border_id

Corder_date

Dproduct_id

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting order_id instead of customer_id in the subquery.

Using a column that does not exist in the customers table.

The subquery must select customer_id from the customers table to match the outer query's customer_id.

2fill in blank

medium

Complete the code to join customers and orders tables to get customer names and their order dates.

SQL

SELECT c.name, o.order_date FROM customers c [1] orders o ON c.customer_id = o.customer_id;

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

ALEFT JOIN

BRIGHT JOIN

CINNER JOIN

DFULL JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using LEFT JOIN which includes customers without orders.

Using FULL JOIN which includes unmatched rows from both tables.

An INNER JOIN returns only customers who have orders, matching the requirement.

3fill in blank

hard

Fix the error in the query to find customers who have not placed any orders using a subquery.

SQL

SELECT name FROM customers WHERE customer_id [1] (SELECT customer_id FROM orders);

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

AIN

BNOT EXISTS

CEXISTS

DNOT IN

Attempts:

3 left

💡 Hint

Common Mistakes

Using IN instead of NOT IN, which returns customers with orders.

Using EXISTS incorrectly without a correlated subquery.

To find customers without orders, use NOT IN to exclude those with matching orders.

4fill in blank

hard

Fill both blanks to write a query that lists customers and their order counts, including customers with zero orders.

SQL

SELECT c.name, COUNT(o.order_id) AS order_count FROM customers c [1] JOIN orders o ON c.customer_id [2] o.customer_id GROUP BY c.name;

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

ALEFT

BRIGHT

C=

D<>

Attempts:

3 left

💡 Hint

Common Mistakes

Using RIGHT JOIN which may exclude some customers.

Using <> instead of = in the join condition.

A LEFT JOIN includes all customers even if they have no orders. The join condition uses = to match customer IDs.

5fill in blank

hard

Fill all three blanks to write a query that selects customers with orders after 2023-01-01 using a JOIN and a WHERE filter.

SQL

SELECT c.name, o.order_date FROM customers c [1] JOIN orders o ON c.customer_id [2] o.customer_id WHERE o.order_date [3] '2023-01-01';

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

AINNER

B=

C>

DLEFT

Attempts:

3 left

💡 Hint

Common Mistakes

Using LEFT JOIN which includes customers without orders.

Using < or = in the date filter instead of >.

An INNER JOIN returns customers with matching orders. The join condition uses = to match IDs. The WHERE clause filters orders after 2023-01-01 using >.

Practice

1. Which statement best describes the performance difference between a JOIN and a subquery in SQL?

easy

A. JOINs generally perform better because they combine tables in a single step.

B. Subqueries always perform better because they run separately.

C. JOINs and subqueries have the same performance in all cases.

D. Subqueries are faster because they use less memory.

Solution

1. Step 1: Understand how JOINs work

   JOINs combine rows from two or more tables in one operation, which is often optimized by the database engine.

2. Step 2: Compare with subqueries

   Subqueries run separately and then feed results to the main query, which can be slower especially with large data.

3. Final Answer:

   JOINs generally perform better because they combine tables in a single step. -> Option A

4. Quick Check:

   JOIN performance > Subquery performance [OK]

Hint: JOINs usually run faster than subqueries [OK]

Common Mistakes:

• Thinking subqueries always run faster
• Assuming JOINs and subqueries are always equal
• Believing subqueries use less memory

2. Which of the following SQL queries correctly uses a JOIN to get all customers and their orders?

easy

A. SELECT customers.name, orders.id FROM customers JOIN orders ON customers.id = orders.customer_id;

B. SELECT customers.name, orders.id FROM customers WHERE customers.id = orders.customer_id;

C. SELECT customers.name, orders.id FROM customers, orders WHERE customers.id == orders.customer_id;

D. SELECT customers.name, orders.id FROM customers JOIN orders ON customers.customer_id = orders.id;

Solution

1. Step 1: Check JOIN syntax

   Correct JOIN syntax uses ON with matching keys: customers.id = orders.customer_id.

2. Step 2: Validate each option

   SELECT customers.name, orders.id FROM customers JOIN orders ON customers.id = orders.customer_id; uses correct JOIN and ON condition. SELECT customers.name, orders.id FROM customers WHERE customers.id = orders.customer_id; uses WHERE without JOIN, which is invalid here. SELECT customers.name, orders.id FROM customers, orders WHERE customers.id == orders.customer_id; uses double equals (==) which is invalid in SQL. SELECT customers.name, orders.id FROM customers JOIN orders ON customers.customer_id = orders.id; reverses keys incorrectly.

3. Final Answer:

   SELECT customers.name, orders.id FROM customers JOIN orders ON customers.id = orders.customer_id; -> Option A

4. Quick Check:

   Correct JOIN syntax = SELECT customers.name, orders.id FROM customers JOIN orders ON customers.id = orders.customer_id; [OK]

Hint: JOIN uses ON with matching keys, not WHERE or == [OK]

Common Mistakes:

• Using WHERE instead of ON for JOIN condition
• Using == instead of = in SQL
• Mixing up key columns in ON clause

3. Given the tables employees(id, name) and departments(id, name, manager_id), what will this query return?

SELECT e.name FROM employees e WHERE e.id IN (SELECT d.manager_id FROM departments d);

medium

A. Syntax error due to subquery.

B. Names of all employees regardless of department.

C. Names of employees who are not managers.

D. Names of employees who are managers of any department.

Solution

1. Step 1: Understand the subquery

   The subquery SELECT d.manager_id FROM departments d returns all manager IDs from departments.

2. Step 2: Analyze the main query

   The main query selects employee names where their ID is in the list of manager IDs, so it returns employees who manage departments.

3. Final Answer:

   Names of employees who are managers of any department. -> Option D

4. Quick Check:

   Subquery filters managers = Names of employees who are managers of any department. [OK]

Hint: IN with subquery filters matching IDs [OK]

Common Mistakes:

• Thinking it returns all employees
• Confusing managers with non-managers
• Assuming syntax error in subquery

4. Identify the error in this SQL query that uses a JOIN:

SELECT c.name, o.amount FROM customers c JOIN orders o WHERE c.id = o.customer_id;

medium

A. Incorrect table aliases used.

B. Using WHERE instead of HAVING for condition.

C. Missing ON keyword before join condition.

D. No error; query is correct.

Solution

1. Step 1: Review JOIN syntax

   JOIN requires an ON clause to specify join condition, not WHERE.

2. Step 2: Check the query

   The query uses WHERE for join condition, which is incorrect syntax for explicit JOIN.

3. Final Answer:

   Missing ON keyword before join condition. -> Option C

4. Quick Check:

   JOIN needs ON, not WHERE [OK]

Hint: JOIN must have ON clause for conditions [OK]

Common Mistakes:

• Using WHERE instead of ON for JOIN
• Confusing HAVING with WHERE
• Assuming aliases cause error

5. You want to list all products and their category names. The products table has category_id, and the categories table has id and name. Which approach is better for performance and why?

Options:
A) Use a JOIN to combine products and categories.
B) Use a subquery in SELECT to get category name for each product.
C) Use a subquery in WHERE to filter products by category name.
D) Use UNION to combine products and categories.

hard

A. Subquery in SELECT is better because it runs once per product.

B. JOIN is better because it retrieves all data in one step efficiently.

C. Subquery in WHERE is better because it filters early.

D. UNION is better because it merges tables.

Solution

1. Step 1: Understand the data retrieval goal

   You want product info with category names, which requires combining data from two tables.

2. Step 2: Compare approaches

   JOIN combines tables in one efficient operation. Subqueries in SELECT run once per row, causing slower performance. Subquery in WHERE filters but doesn't retrieve category names. UNION merges rows, not related here.

3. Final Answer:

   JOIN is better because it retrieves all data in one step efficiently. -> Option B

4. Quick Check:

   JOIN efficiency > subqueries for this task [OK]

Hint: JOIN combines tables efficiently for related data [OK]

Common Mistakes:

• Using subquery in SELECT causing slow per-row lookup
• Confusing UNION with JOIN
• Using subquery in WHERE without retrieving needed data