Why aggregation summarizes data in MySQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to count the number of employees in the table.

MySQL

SELECT COUNT([1]) FROM employees;

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

Aname

Bemployee_id

C*

Dsalary

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT with a specific column may miss NULL values.

Forgetting to use COUNT function.

The COUNT(*) function counts all rows in the table, summarizing the total number of employees.

2fill in blank

medium

Complete the code to find the average salary of employees.

MySQL

SELECT AVG([1]) FROM employees;

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

Aemployee_id

Bsalary

Cname

Ddepartment

Attempts:

3 left

💡 Hint

Common Mistakes

Using AVG on non-numeric columns.

Using SUM instead of AVG.

The AVG(salary) function calculates the average salary, summarizing the salary data.

3fill in blank

hard

Fix the error in the code to get the total salary paid to employees.

MySQL

SELECT [1](salary) FROM employees;

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

ASUM

BCOUNT

CAVG

DMAX

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT instead of SUM.

Using AVG when total sum is needed.

The SUM(salary) function adds all salaries together, summarizing total payment.

4fill in blank

hard

Fill both blanks to find the maximum and minimum salaries.

MySQL

SELECT [1](salary) AS max_salary, [2](salary) AS min_salary FROM employees;

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

AMAX

BMIN

CSUM

DAVG

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up MAX and MIN functions.

Using SUM or AVG instead of MAX or MIN.

MAX(salary) finds the highest salary, and MIN(salary) finds the lowest salary, summarizing salary range.

5fill in blank

hard

Fill all three blanks to get the count, average, and total salary for employees.

MySQL

SELECT [1](*) AS total_employees, [2](salary) AS average_salary, [3](salary) AS total_salary FROM employees;

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

ACOUNT

BAVG

CSUM

DMAX

Attempts:

3 left

💡 Hint

Common Mistakes

Using MAX instead of SUM for total salary.

Using COUNT on salary instead of COUNT(*).

This query counts employees, calculates average salary, and sums total salary, summarizing key data points.

Practice

1. What is the main purpose of aggregation functions like SUM and COUNT in SQL?

easy

A. To create new tables from existing ones

B. To delete duplicate rows from a table

C. To combine multiple rows into a single summary value

D. To sort rows in ascending or descending order

Solution

1. Step 1: Understand aggregation functions

   Aggregation functions like SUM and COUNT take many rows and produce one value summarizing them.

2. Step 2: Identify the purpose of aggregation

   They are used to combine data, not to delete, create tables, or sort rows.

3. Final Answer:

   To combine multiple rows into a single summary value -> Option C

4. Quick Check:

   Aggregation = summary value [OK]

Hint: Aggregation means summarizing many rows into one value [OK]

Common Mistakes:

• Confusing aggregation with sorting
• Thinking aggregation deletes rows
• Believing aggregation creates new tables

2. Which of the following is the correct syntax to calculate the average salary from a table named employees?

easy

A. SELECT AVG(salary) FROM employees;

B. SELECT AVERAGE(salary) FROM employees;

C. SELECT MEAN(salary) FROM employees;

D. SELECT SUM(salary) / COUNT(*) FROM employees GROUP BY salary;

Solution

1. Step 1: Recall standard SQL aggregation functions

   The correct function for average is AVG, not AVERAGE or MEAN.

2. Step 2: Check syntax correctness

   SELECT AVG(salary) FROM employees; uses the correct AVG function with proper syntax. Options B and C use non-standard functions. SELECT SUM(salary) / COUNT(*) FROM employees GROUP BY salary; uses manual SUM/COUNT but GROUP BY salary computes per-salary value instead of overall average.

3. Final Answer:

   SELECT AVG(salary) FROM employees; -> Option A

4. Quick Check:

   AVG() is standard average function [OK]

Hint: Use AVG() for average, not AVERAGE or MEAN [OK]

Common Mistakes:

• Using non-existent functions like AVERAGE or MEAN
• Trying to calculate average manually instead of using AVG()
• Missing semicolon at the end

3. Given the table sales with columns region and amount, what will this query return?

SELECT region, SUM(amount) FROM sales GROUP BY region;

medium

A. Total sales amount for each region

B. List of all sales amounts without grouping

C. Average sales amount for all regions combined

D. Number of sales records in the table

Solution

1. Step 1: Understand GROUP BY with SUM

   The query groups rows by region and sums the amount in each group.

2. Step 2: Interpret the result

   It returns one row per region with the total sales amount for that region.

3. Final Answer:

   Total sales amount for each region -> Option A

4. Quick Check:

   GROUP BY region + SUM(amount) = total per region [OK]

Hint: GROUP BY groups rows; SUM adds values per group [OK]

Common Mistakes:

• Thinking it returns all individual sales
• Confusing SUM with AVG
• Ignoring GROUP BY effect

4. Identify the error in this query:

SELECT department, COUNT(employee_id) FROM employees;

medium

A. COUNT cannot be used with employee_id

B. Missing GROUP BY clause for department

C. SELECT must include WHERE clause

D. employee_id should be inside SUM()

Solution

1. Step 1: Check aggregation with non-aggregated column

   The query selects department (non-aggregated) and COUNT(employee_id) (aggregated) without GROUP BY.

2. Step 2: Understand SQL rules for aggregation

   When mixing aggregated and non-aggregated columns, GROUP BY is required on non-aggregated columns.

3. Final Answer:

   Missing GROUP BY clause for department -> Option B

4. Quick Check:

   Non-aggregated columns need GROUP BY [OK]

Hint: Use GROUP BY when selecting non-aggregated columns with aggregation [OK]

Common Mistakes:

• Omitting GROUP BY with mixed columns
• Misusing COUNT on non-numeric columns
• Adding unnecessary WHERE clause

5. You want to find the department with the highest average salary from the employees table. Which query correctly summarizes this data?

hard

A. SELECT department, MAX(AVG(salary)) FROM employees GROUP BY department;

B. SELECT department, SUM(salary) FROM employees;

C. SELECT MAX(salary) FROM employees GROUP BY department;

D. SELECT department, AVG(salary) FROM employees GROUP BY department ORDER BY AVG(salary) DESC LIMIT 1;

Solution

1. Step 1: Understand the goal

   We want the department with the highest average salary, so we need average per department and then pick the top one.

2. Step 2: Analyze each option

   SELECT department, MAX(AVG(salary)) FROM employees GROUP BY department; is invalid because MAX(AVG(salary)) is not allowed directly. SELECT department, AVG(salary) FROM employees GROUP BY department ORDER BY AVG(salary) DESC LIMIT 1; correctly groups by department, calculates average salary, orders descending, and limits to one result. SELECT MAX(salary) FROM employees GROUP BY department; finds max salary per department, not average. SELECT department, SUM(salary) FROM employees; sums salary but does not group or find max.

3. Final Answer:

   SELECT department, AVG(salary) FROM employees GROUP BY department ORDER BY AVG(salary) DESC LIMIT 1; -> Option D

4. Quick Check:

   Group + AVG + ORDER BY + LIMIT = highest average [OK]

Hint: Use GROUP BY, AVG, ORDER BY DESC, LIMIT 1 for top average [OK]

Common Mistakes:

• Trying to nest aggregation functions incorrectly
• Not ordering results to find highest value
• Missing GROUP BY when aggregating