Scalar subqueries in MySQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the name and the highest salary from the employees table using a scalar subquery.

MySQL

SELECT name, (SELECT [1] FROM employees) AS max_salary FROM employees;

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

ACOUNT(salary)

BMAX(salary)

Csalary

DMIN(salary)

Attempts:

3 left

💡 Hint

Common Mistakes

Using salary without aggregation returns multiple rows causing an error.

Using COUNT(salary) returns the number of salaries, not the highest salary.

The scalar subquery must return the maximum salary, so MAX(salary) is correct.

2fill in blank

medium

Complete the code to find employees whose salary is equal to the average salary using a scalar subquery.

MySQL

SELECT name FROM employees WHERE salary = (SELECT [1] FROM employees);

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

AAVG(salary)

BMIN(salary)

CSUM(salary)

DMAX(salary)

Attempts:

3 left

💡 Hint

Common Mistakes

Using MAX(salary) or MIN(salary) returns wrong comparison values.

Using SUM(salary) returns the total salary, not average.

The scalar subquery should return the average salary, so AVG(salary) is correct.

3fill in blank

hard

Fix the error in the scalar subquery to correctly find employees with salary greater than the average salary.

MySQL

SELECT name FROM employees WHERE salary > (SELECT [1]salary FROM employees);

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

AMAX(

BFROM

CAVG(

DMIN(

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting AVG() parentheses causes syntax errors.

Using FROM without an aggregate function is invalid here.

The scalar subquery must use AVG(salary) to get the average salary for comparison.

4fill in blank

hard

Fill both blanks to select employees whose salary is between the minimum and maximum salary using scalar subqueries.

MySQL

SELECT name FROM employees WHERE salary BETWEEN (SELECT [1](salary) FROM employees) AND (SELECT [2](salary) FROM employees);

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

AMIN

BMAX

CAVG

DCOUNT

Attempts:

3 left

💡 Hint

Common Mistakes

Using AVG() or COUNT() returns incorrect ranges.

Swapping MIN and MAX changes the logic.

The salary should be between the minimum and maximum salaries, so MIN and MAX are correct.

5fill in blank

hard

Fill all three blanks to select employees whose salary is greater than the average salary but less than the maximum salary.

MySQL

SELECT name FROM employees WHERE salary > (SELECT [1](salary) FROM employees) AND salary < (SELECT [2](salary) FROM employees) ORDER BY [3];

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

AMIN

BAVG

Csalary

DMAX

Attempts:

3 left

💡 Hint

Common Mistakes

Using MIN() instead of AVG() or MAX() causes wrong filtering.

Ordering by an aggregate function instead of a column causes errors.

The query compares salary to average and maximum salaries, so AVG and MAX are correct. Ordering by salary sorts the results by salary.

Practice

1. What does a scalar subquery return in MySQL?

easy

A. A list of tables

B. A single value that can be used in another query

C. Only column names

D. Multiple rows and columns

Solution

1. Step 1: Understand scalar subquery definition

   A scalar subquery returns exactly one value, not multiple rows or columns.

2. Step 2: Identify usage in queries

   This single value can be used inside another query, like in WHERE or SELECT clauses.

3. Final Answer:

   A single value that can be used in another query -> Option B

4. Quick Check:

   Scalar subquery = single value [OK]

Hint: Scalar subqueries always return one value only [OK]

Common Mistakes:

• Thinking scalar subqueries return multiple rows
• Confusing scalar subqueries with table subqueries
• Assuming scalar subqueries return column names only

2. Which of the following is the correct syntax for a scalar subquery in a SELECT statement?

easy

A. SELECT name, MAX(score) FROM scores FROM students;

B. SELECT name, MAX(score) AS max_score WHERE score > 50;

C. SELECT name, (SELECT MAX(score) FROM scores) AS max_score FROM students;

D. SELECT name, (MAX(score) FROM scores) AS max_score FROM students;

Solution

1. Step 1: Analyze correct scalar subquery syntax

   The scalar subquery must be enclosed in parentheses and return a single value, as in (SELECT MAX(score) FROM scores).

2. Step 2: Check placement in SELECT clause

   SELECT name, (SELECT MAX(score) FROM scores) AS max_score FROM students; correctly places the scalar subquery inside the SELECT list with an alias.

3. Final Answer:

   SELECT name, (SELECT MAX(score) FROM scores) AS max_score FROM students; -> Option C

4. Quick Check:

   Scalar subquery syntax = SELECT name, (SELECT MAX(score) FROM scores) AS max_score FROM students; [OK]

Hint: Scalar subqueries must be in parentheses inside SELECT or WHERE [OK]

Common Mistakes:

• Missing parentheses around subquery
• Incorrect FROM clause placement
• Using aggregate functions without subquery

3. Given the tables:
students(id, name)
scores(student_id, score)
What will this query return?

SELECT name, (SELECT AVG(score) FROM scores WHERE student_id = students.id) AS avg_score FROM students;

medium

A. Each student's name with their average score

B. The overall average score for all students repeated for each name

C. Syntax error due to subquery

D. List of student names only without scores

Solution

1. Step 1: Understand the subquery correlation

   The subquery calculates the average score for each student by matching student_id with students.id.

2. Step 2: Result of the main query

   The main query returns each student's name along with their individual average score from the subquery.

3. Final Answer:

   Each student's name with their average score -> Option A

4. Quick Check:

   Scalar subquery returns per-student average [OK]

Hint: Correlated scalar subqueries return values per row [OK]

Common Mistakes:

• Assuming subquery returns overall average for all rows
• Confusing correlated and uncorrelated subqueries
• Expecting syntax error when none exists

4. Identify the error in this query using a scalar subquery:

SELECT name, (SELECT score FROM scores) AS score FROM students;

medium

A. No error, query runs fine

B. Missing alias for subquery

C. Incorrect table name in subquery

D. Subquery returns multiple rows causing an error

Solution

1. Step 1: Analyze the subquery result

   The subquery SELECT score FROM scores returns multiple rows because no WHERE clause limits it.

2. Step 2: Understand scalar subquery requirement

   Scalar subqueries must return exactly one value; multiple rows cause an error.

3. Final Answer:

   Subquery returns multiple rows causing an error -> Option D

4. Quick Check:

   Scalar subquery must return one value [OK]

Hint: Scalar subqueries must return one value, not many [OK]

Common Mistakes:

• Ignoring multiple rows returned by subquery
• Assuming alias is mandatory for subquery
• Thinking query runs without error

5. You want to find students whose score is above the average score of all students. Which query correctly uses a scalar subquery to achieve this?

hard

A. SELECT name FROM students WHERE score > (SELECT AVG(score) FROM scores);

B. SELECT name FROM students WHERE score > SELECT AVG(score) FROM scores;

C. SELECT name FROM students WHERE (SELECT AVG(score) FROM scores) < (SELECT score FROM scores WHERE student_id = students.id);

D. SELECT name FROM students WHERE score > (SELECT score FROM scores);

Solution

1. Step 1: Understand the goal

   We want students whose individual score is greater than the average score of all students.

2. Step 2: Analyze each option

   SELECT name FROM students WHERE score > (SELECT AVG(score) FROM scores); correctly compares each student's score with the average score using a scalar subquery.

3. Final Answer:

   SELECT name FROM students WHERE score > (SELECT AVG(score) FROM scores); -> Option A

4. Quick Check:

   Scalar subquery compares average with individual score [OK]

Hint: Use scalar subquery for average, correlate for individual score [OK]

Common Mistakes:

• Missing parentheses around subqueries
• Using subquery without correlation for individual scores
• Syntax errors from missing SELECT in WHERE