LATERAL join for correlated subqueries in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select each department and its employee count using LATERAL join.

PostgreSQL

SELECT d.department_name, emp_count.count FROM departments d LEFT JOIN LATERAL (SELECT COUNT(*) AS count FROM employees e WHERE e.department_id = d.id) AS emp_count ON [1];

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

Ad.id = e.department_id

Bemp_count.count > 0

Ctrue

Dd.department_name = e.name

Attempts:

3 left

💡 Hint

Common Mistakes

Using a join condition that references tables not in scope in the ON clause.

Leaving the ON clause empty or invalid.

The LATERAL join requires a condition after ON. Using true means the join always applies, allowing the subquery to correlate with the outer query.

2fill in blank

medium

Complete the code to select each employee and their latest project using LATERAL join.

PostgreSQL

SELECT e.name, p.project_name FROM employees e LEFT JOIN LATERAL (SELECT project_name FROM projects p WHERE p.employee_id = e.id ORDER BY p.start_date DESC LIMIT 1) AS p ON [1];

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

Ap.employee_id = e.id

Btrue

Ce.id = p.id

Dp.project_name IS NOT NULL

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to join on columns that do not exist in the subquery alias.

Using a join condition that filters out rows unintentionally.

The ON clause for LATERAL join can be true to always join the subquery result for each employee.

3fill in blank

hard

Fix the error in the LATERAL join by completing the ON clause correctly.

PostgreSQL

SELECT d.name, emp.name FROM departments d JOIN LATERAL (SELECT name FROM employees emp WHERE emp.department_id = d.id LIMIT 1) emp ON [1];

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

Aemp.id = d.id

Bemp.department_id = d.id

Cemp.name = d.name

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using join conditions that reference columns not available in the lateral subquery alias.

Leaving the ON clause empty.

The ON clause in a LATERAL join can be true to allow the lateral subquery to run correlated with the outer query.

4fill in blank

hard

Fill both blanks to create a LATERAL join that selects each customer and their most recent order date.

PostgreSQL

SELECT c.customer_name, recent_orders.order_date FROM customers c LEFT JOIN LATERAL (SELECT order_date FROM orders o WHERE o.customer_id = c.id ORDER BY order_date DESC LIMIT 1) AS recent_orders ON [1] [2];

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

Atrue

Bo.customer_id = c.id

CAND

DOR

Attempts:

3 left

💡 Hint

Common Mistakes

Using invalid logical operators like OR without proper conditions.

Trying to join on columns not available in the lateral subquery alias.

The ON clause can be true for LATERAL joins. If combining conditions, use AND or OR. Here, true AND is syntactically valid but usually only true is needed.

5fill in blank

hard

Fill all three blanks to write a query that selects each product and its top 3 reviews using LATERAL join.

PostgreSQL

SELECT p.product_name, r.review_text FROM products p JOIN LATERAL (SELECT review_text FROM reviews r WHERE r.product_id = p.id ORDER BY r.rating DESC LIMIT [1]) r ON [2] [3];

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

A3

Btrue

CAND

Dr.product_id = p.id

Attempts:

3 left

💡 Hint

Common Mistakes

Using LIMIT values other than 3.

Using join conditions that cause errors or filter out rows incorrectly.

The LIMIT is 3 to get top 3 reviews. The ON clause can be true. If combining conditions, use AND. Usually, just true is enough for LATERAL joins.

Practice

1. What is the main purpose of using a LATERAL join in PostgreSQL?

easy

A. To run a subquery for each row of the main table using that row's data

B. To join two tables without any condition

C. To create a temporary table for all rows at once

D. To speed up queries by caching results

Solution

1. Step 1: Understand what LATERAL join does

   A LATERAL join allows a subquery to use columns from the current row of the main query.

2. Step 2: Identify the main purpose

   This means the subquery runs once per row, using that row's data to produce related results.

3. Final Answer:

   To run a subquery for each row of the main table using that row's data -> Option A

4. Quick Check:

   LATERAL join = per-row subquery execution [OK]

Hint: LATERAL runs subqueries per row using that row's data [OK]

Common Mistakes:

• Thinking LATERAL joins all rows at once
• Confusing LATERAL with regular JOIN
• Assuming LATERAL speeds up queries by caching

2. Which of the following is the correct syntax to use a LATERAL join in PostgreSQL?

easy

A. SELECT a.id, b.value FROM table_a a JOIN (SELECT value FROM table_b WHERE b.a_id = a.id) b ON true;

B. SELECT a.id, b.value FROM table_a a JOIN LATERAL (SELECT value FROM table_b WHERE b.a_id = a.id) b ON true;

C. SELECT a.id, b.value FROM table_a a LEFT JOIN LATERAL table_b b ON b.a_id = a.id;

D. SELECT a.id, b.value FROM table_a a CROSS JOIN table_b b WHERE b.a_id = a.id;

Solution

1. Step 1: Identify correct LATERAL syntax

   LATERAL must be explicitly stated before the subquery or table function to allow referencing outer query columns.

2. Step 2: Check each option

   SELECT a.id, b.value FROM table_a a JOIN LATERAL (SELECT value FROM table_b WHERE b.a_id = a.id) b ON true; correctly uses JOIN LATERAL with a subquery referencing outer alias 'a'. SELECT a.id, b.value FROM table_a a JOIN (SELECT value FROM table_b WHERE b.a_id = a.id) b ON true; misses LATERAL keyword. SELECT a.id, b.value FROM table_a a LEFT JOIN LATERAL table_b b ON b.a_id = a.id; uses LATERAL but tries to join a table directly without subquery syntax. SELECT a.id, b.value FROM table_a a CROSS JOIN table_b b WHERE b.a_id = a.id; uses CROSS JOIN without LATERAL, so no correlation.

3. Final Answer:

   SELECT a.id, b.value FROM table_a a JOIN LATERAL (SELECT value FROM table_b WHERE b.a_id = a.id) b ON true; -> Option B

4. Quick Check:

   Correct LATERAL syntax includes keyword before subquery [OK]

Hint: LATERAL keyword must precede subquery to access outer columns [OK]

Common Mistakes:

• Omitting LATERAL keyword before subquery
• Using regular JOIN without correlation
• Trying to join tables directly without subquery

3. Given the tables:
users(id, name)
orders(id, user_id, amount)
What will this query return?

SELECT u.name, o.amount FROM users u JOIN LATERAL (SELECT amount FROM orders WHERE user_id = u.id ORDER BY amount DESC LIMIT 1) o ON true;

medium

A. Users with all their orders listed

B. All orders with user names repeated

C. Syntax error due to missing ON condition

D. Each user with their highest order amount

Solution

1. Step 1: Understand the LATERAL subquery

   The subquery selects the highest order amount per user by ordering descending and limiting to 1.

2. Step 2: Analyze the JOIN LATERAL effect

   For each user row, the subquery runs and returns that user's top order amount, joining on true to include all users with orders.

3. Final Answer:

   Each user with their highest order amount -> Option D

4. Quick Check:

   LATERAL subquery returns top order per user [OK]

Hint: LATERAL runs subquery per user to get top order [OK]

Common Mistakes:

• Thinking it returns all orders per user
• Assuming syntax error due to ON true
• Confusing join with CROSS JOIN

4. Identify the error in this query:

SELECT u.name, o.amount FROM users u JOIN LATERAL (SELECT amount FROM orders WHERE user_id = u.id ORDER BY amount DESC LIMIT 1) o;

medium

A. Subquery cannot reference outer query columns

B. Incorrect alias usage for orders table

C. Missing ON clause after LATERAL join

D. ORDER BY not allowed in subqueries

Solution

1. Step 1: Check JOIN syntax

   Every JOIN requires an ON clause or USING clause to specify join condition.

2. Step 2: Verify the query

   The query uses JOIN LATERAL but omits ON clause, causing syntax error.

3. Final Answer:

   Missing ON clause after LATERAL join -> Option C

4. Quick Check:

   JOIN must have ON or USING clause [OK]

Hint: Always add ON clause after JOIN LATERAL [OK]

Common Mistakes:

• Forgetting ON clause after JOIN LATERAL
• Thinking subquery can't use outer columns
• Believing ORDER BY is disallowed in subqueries

5. You want to list each customer with their two most recent orders using LATERAL join. Which query correctly achieves this?

hard

A. SELECT c.name, o.order_id FROM customers c JOIN LATERAL (SELECT order_id FROM orders WHERE customer_id = c.id ORDER BY order_date DESC LIMIT 2) o ON true;

B. SELECT c.name, o.order_id FROM customers c JOIN orders o ON o.customer_id = c.id ORDER BY o.order_date DESC LIMIT 2;

C. SELECT c.name, o.order_id FROM customers c CROSS JOIN LATERAL orders o WHERE o.customer_id = c.id LIMIT 2;

D. SELECT c.name, o.order_id FROM customers c LEFT JOIN LATERAL (SELECT order_id FROM orders WHERE customer_id = c.id ORDER BY order_date ASC LIMIT 2) o ON true;

Solution

1. Step 1: Understand the requirement

   We want two most recent orders per customer, so order by order_date DESC and limit 2 in subquery.

2. Step 2: Check each option

   SELECT c.name, o.order_id FROM customers c JOIN LATERAL (SELECT order_id FROM orders WHERE customer_id = c.id ORDER BY order_date DESC LIMIT 2) o ON true; uses JOIN LATERAL with correct ordering and limit, joining on true to include all customers with orders. SELECT c.name, o.order_id FROM customers c JOIN orders o ON o.customer_id = c.id ORDER BY o.order_date DESC LIMIT 2; limits total rows, not per customer. SELECT c.name, o.order_id FROM customers c CROSS JOIN LATERAL orders o WHERE o.customer_id = c.id LIMIT 2; uses CROSS JOIN LATERAL incorrectly without subquery. SELECT c.name, o.order_id FROM customers c LEFT JOIN LATERAL (SELECT order_id FROM orders WHERE customer_id = c.id ORDER BY order_date ASC LIMIT 2) o ON true; orders ASC, which gives oldest orders, not recent.

3. Final Answer:

   SELECT c.name, o.order_id FROM customers c JOIN LATERAL (SELECT order_id FROM orders WHERE customer_id = c.id ORDER BY order_date DESC LIMIT 2) o ON true; -> Option A

4. Quick Check:

   LATERAL with ORDER BY DESC and LIMIT 2 per customer [OK]

Hint: Use LATERAL with ORDER BY DESC and LIMIT for top N per row [OK]

Common Mistakes:

• Limiting total rows instead of per customer
• Ordering ascending instead of descending
• Using CROSS JOIN LATERAL without subquery