Subquery in WHERE clause in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all employees who work in the department with ID 5.

SQL

SELECT * FROM employees WHERE department_id = [1];

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

A5

B10

Cdepartment_id

D'5'

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around the number 5, which makes it a string.

Using the column name instead of a value.

The department ID to filter by is 5, so we use 5 without quotes because it is a number.

2fill in blank

medium

Complete the code to select all employees whose department_id is in the list of department IDs from the departments table.

SQL

SELECT * FROM employees WHERE department_id IN (SELECT [1] FROM departments);

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

Asalary

Bemployee_id

Cname

Ddepartment_id

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting employee_id instead of department_id in the subquery.

Selecting a column unrelated to departments.

The subquery should select department_id from the departments table to match with employees' department_id.

3fill in blank

hard

Fix the error in the query to select employees who have a salary greater than the average salary in their department.

SQL

SELECT * FROM employees WHERE salary > (SELECT AVG([1]) FROM employees WHERE department_id = employees.department_id);

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

Aemployee_id

B*

Csalary

Ddepartment_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using department_id inside AVG, which is not numeric for averaging.

Using * inside AVG, which is invalid.

The subquery calculates the average salary, so it must use the salary column.

4fill in blank

hard

Fill both blanks to select all products with a price higher than the average price in their category.

SQL

SELECT * FROM products WHERE price > (SELECT [1] FROM products WHERE category_id = [2]);

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

AAVG(price)

Bproducts.category_id

Cdepartment_id

DMAX(price)

Attempts:

3 left

💡 Hint

Common Mistakes

Using MAX(price) instead of AVG(price).

Using department_id instead of products.category_id.

The subquery calculates the average price, so use AVG(price). The category filter uses products.category_id to match the outer query.

5fill in blank

hard

Fill all three blanks to select employees whose salary is greater than the average salary in their department and whose department name is 'Sales'.

SQL

SELECT * FROM employees WHERE salary > (SELECT AVG([1]) FROM employees WHERE department_id = [2]) AND department_id = (SELECT id FROM departments WHERE name = [3]);

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

Asalary

Bemployees.department_id

C'Sales'

Ddepartment_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using department_id without table prefix in the subquery, causing ambiguity.

Using Sales without quotes, causing syntax error.

Using wrong column names in AVG or subqueries.

The first blank is the salary column for averaging. The second blank must reference the outer employees table's department_id. The third blank is the string 'Sales' to match the department name.

Practice

1. What does a subquery in the WHERE clause do in SQL?

easy

A. Creates a new table from existing data

B. Filters rows based on the results of another query

C. Deletes rows from a table

D. Updates values in a table

Solution

1. Step 1: Understand the role of subqueries in WHERE clause

   A subquery inside a WHERE clause is used to filter rows by comparing values to the results of another query.

2. Step 2: Compare with other SQL operations

   Creating tables, deleting, or updating rows are different SQL operations and not the purpose of subqueries in WHERE.

3. Final Answer:

   Filters rows based on the results of another query -> Option B

4. Quick Check:

   Subquery in WHERE = filter rows [OK]

Hint: Subquery in WHERE filters rows using another query's results [OK]

Common Mistakes:

• Thinking subquery creates or modifies tables
• Confusing subquery with JOIN
• Assuming subquery always returns one value

2. Which of the following is the correct syntax to use a subquery in the WHERE clause to find employees in departments with ID 10 or 20?

easy

A. SELECT * FROM employees WHERE department_id IN SELECT department_id FROM departments WHERE department_id IN (10, 20);

B. SELECT * FROM employees WHERE department_id = (SELECT department_id FROM departments WHERE department_id IN (10, 20));

C. SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE department_id = 10 OR 20);

D. SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE department_id IN (10, 20));

Solution

1. Step 1: Identify correct subquery syntax with IN

   The subquery must be enclosed in parentheses and used with IN to match multiple values.

2. Step 2: Check each option's syntax

   SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE department_id IN (10, 20)); correctly uses IN with a subquery in parentheses. SELECT * FROM employees WHERE department_id = (SELECT department_id FROM departments WHERE department_id IN (10, 20)); uses = which expects one value, causing error. SELECT * FROM employees WHERE department_id IN SELECT department_id FROM departments WHERE department_id IN (10, 20); misses parentheses around subquery. SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE department_id = 10 OR 20); has incorrect WHERE clause syntax.

3. Final Answer:

   SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE department_id IN (10, 20)); -> Option D

4. Quick Check:

   Subquery with IN needs parentheses [OK]

Hint: Use IN with parentheses for subqueries returning multiple values [OK]

Common Mistakes:

• Using = instead of IN for multiple values
• Omitting parentheses around subquery
• Incorrect WHERE clause conditions inside subquery

3. Given the tables:
employees(emp_id, name, department_id)
departments(department_id, name)
What will this query return?

SELECT name FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE name = 'Sales');

medium

A. Names of employees who work in the Sales department

B. Names of all employees

C. Names of departments named Sales

D. An error because subquery returns multiple rows

Solution

1. Step 1: Understand the subquery

   The subquery selects department_id from departments where name is 'Sales'. This returns IDs of Sales departments.

2. Step 2: Apply subquery results in main query

   The main query selects employee names where their department_id matches any of those returned by the subquery.

3. Final Answer:

   Names of employees who work in the Sales department -> Option A

4. Quick Check:

   Subquery filters employees by Sales department [OK]

Hint: Subquery filters department IDs, main query filters employees [OK]

Common Mistakes:

• Thinking subquery returns employee names
• Confusing department names with employee names
• Assuming subquery causes error with multiple rows

4. Identify the error in this query:

SELECT * FROM orders WHERE customer_id = (SELECT customer_id FROM customers WHERE city = 'New York');

medium

A. Incorrect table name 'orders'

B. Missing FROM clause in subquery

C. Subquery returns multiple rows causing an error with '=' operator

D. No error, query is correct

Solution

1. Step 1: Analyze subquery result

   The subquery selects customer_id from customers where city is 'New York'. This can return multiple customer IDs.

2. Step 2: Check operator compatibility

   The main query uses '=' which expects a single value, but subquery returns multiple rows, causing an error.

3. Final Answer:

   Subquery returns multiple rows causing an error with '=' operator -> Option C

4. Quick Check:

   Use IN for multiple subquery results [OK]

Hint: Use IN if subquery returns multiple values, not = [OK]

Common Mistakes:

• Using = with subquery returning multiple rows
• Assuming subquery always returns one value
• Ignoring error messages about subquery results

5. You want to find all products that have never been ordered. Given tables:
products(product_id, name)
orders(order_id, product_id)
Which query correctly uses a subquery in the WHERE clause to find these products?

hard

A. SELECT name FROM products WHERE product_id NOT IN (SELECT product_id FROM orders);

B. SELECT name FROM products WHERE product_id IN (SELECT product_id FROM orders);

C. SELECT name FROM products WHERE product_id = (SELECT product_id FROM orders);

D. SELECT name FROM products WHERE product_id NOT = (SELECT product_id FROM orders);

Solution

1. Step 1: Understand the goal

   We want products that have never been ordered, so their product_id should NOT appear in orders.

2. Step 2: Use NOT IN with subquery

   The subquery selects all product_ids from orders. Using NOT IN filters products not in that list.

3. Step 3: Check other options

   SELECT name FROM products WHERE product_id IN (SELECT product_id FROM orders); finds products that have been ordered (opposite). SELECT name FROM products WHERE product_id = (SELECT product_id FROM orders); uses = which expects one value, causing error. SELECT name FROM products WHERE product_id NOT = (SELECT product_id FROM orders); uses NOT = which is invalid syntax for multiple values.

4. Final Answer:

   SELECT name FROM products WHERE product_id NOT IN (SELECT product_id FROM orders); -> Option A

5. Quick Check:

   NOT IN filters products never ordered [OK]

Hint: Use NOT IN with subquery to find missing matches [OK]

Common Mistakes:

• Using = instead of IN for multiple values
• Confusing NOT IN with NOT =
• Selecting wrong table columns in subquery