How GROUP BY changes query execution in SQL - Practice Exercises

Challenge - 5 Problems

❓ query_result

intermediate

Output of GROUP BY with aggregate function

Given the table Sales with columns Region and Amount, what is the output of this query?

SELECT Region, SUM(Amount) FROM Sales GROUP BY Region;

SQL

CREATE TABLE Sales (Region VARCHAR(10), Amount INT);
INSERT INTO Sales VALUES ('East', 100), ('West', 200), ('East', 150), ('West', 50);

A SyntaxError

B [{"Region": "East", "SUM(Amount)": 100}, {"Region": "West", "SUM(Amount)": 200}, {"Region": "East", "SUM(Amount)": 150}, {"Region": "West", "SUM(Amount)": 50}]

C [{"Region": "East", "SUM(Amount)": 250}, {"Region": "West", "SUM(Amount)": 250}]

D [{"Region": null, "SUM(Amount)": 500}]

💡 Hint

GROUP BY groups rows by the column values before applying aggregate functions.

The query groups rows by Region and sums Amount for each group. East has 100 + 150 = 250, West has 200 + 50 = 250.

🧠 Conceptual

intermediate

Effect of GROUP BY on non-aggregated columns

What happens if you select a column in a query with GROUP BY that is not included in the GROUP BY clause or an aggregate function?

A The query returns the first value of that column for each group.

B The query returns an error because all selected columns must be grouped or aggregated.

C The query returns the last value of that column for each group.

D The query ignores the column and returns only grouped columns.

💡 Hint

Think about SQL rules for grouping and selecting columns.

SQL requires that all selected columns must be either in the GROUP BY clause or used inside an aggregate function. Otherwise, it raises an error.

📝 Syntax

advanced

Identify the syntax error in GROUP BY usage

Which option contains a syntax error in the GROUP BY clause?

SQL

Table: Employees (Department VARCHAR, Salary INT)

A SELECT Department, Salary FROM Employees GROUP BY Department;

B SELECT Department, AVG(Salary) FROM Employees GROUP BY Department;

C SELECT Department, COUNT(*) FROM Employees GROUP BY Department;

D SELECT Department, MAX(Salary) FROM Employees GROUP BY Department;

💡 Hint

Check if all selected columns are grouped or aggregated.

Option A selects Salary without aggregation or grouping, causing a syntax error.

❓ optimization

advanced

How GROUP BY affects query performance

Which statement best explains how adding a GROUP BY clause affects query execution?

A GROUP BY has no effect on query execution time.

B GROUP BY always makes queries faster by reducing the number of rows returned.

C GROUP BY eliminates the need for indexes on the grouped columns.

D GROUP BY causes the database to sort or hash rows to group them, which can increase execution time.

💡 Hint

Think about what the database must do to group rows.

Grouping requires sorting or hashing rows, which adds processing overhead and can slow queries.

🔧 Debug

expert

Diagnose the cause of unexpected GROUP BY results

A query:

SELECT Department, COUNT(EmployeeID) FROM Employees GROUP BY Department;

returns lower counts than expected. Which issue is most likely causing this?

A COUNT(EmployeeID) counts only non-NULL EmployeeID, so some rows are excluded.

B Some Department values are NULL, causing those rows to be grouped together.

C The GROUP BY clause is missing a column, causing rows to merge incorrectly.

D The query is missing a WHERE clause to filter rows.

💡 Hint

Consider how COUNT behaves with NULL values.

COUNT(column) counts only rows where the column is not NULL, so rows with NULL EmployeeID are not counted, reducing totals.

Practice

1. What does the GROUP BY clause do in an SQL query?

easy

A. It filters rows based on a condition.

B. It groups rows that have the same values in specified columns.

C. It deletes duplicate rows from the result.

D. It sorts the rows in ascending order.

Solution

1. Step 1: Understand the purpose of GROUP BY

   The GROUP BY clause collects rows with the same values in specified columns into groups.

2. Step 2: Compare with other SQL clauses

   Sorting is done by ORDER BY, filtering by WHERE, and removing duplicates by DISTINCT, not GROUP BY.

3. Final Answer:

   It groups rows that have the same values in specified columns. -> Option B

4. Quick Check:

   GROUP BY groups rows by column values [OK]

Hint: GROUP BY groups rows by column values, not sorting or filtering [OK]

Common Mistakes:

• Confusing GROUP BY with ORDER BY
• Thinking GROUP BY filters rows
• Assuming GROUP BY removes duplicates

2. Which of the following is the correct syntax to group rows by the column department?

easy

A. SELECT department, COUNT(*) FROM employees WHERE department;

B. SELECT department, COUNT(*) FROM employees ORDER BY department;

C. SELECT department, COUNT(*) FROM employees GROUP BY department;

D. SELECT department, COUNT(*) FROM employees HAVING department;

Solution

1. Step 1: Identify correct GROUP BY usage

   The GROUP BY clause must follow the FROM clause and specify the column to group by, here 'department'.

2. Step 2: Check each option's syntax

   SELECT department, COUNT(*) FROM employees GROUP BY department; uses GROUP BY correctly. SELECT department, COUNT(*) FROM employees ORDER BY department; uses ORDER BY which sorts, not groups. SELECT department, COUNT(*) FROM employees WHERE department; uses WHERE incorrectly. SELECT department, COUNT(*) FROM employees HAVING department; uses HAVING without GROUP BY, which is invalid.

3. Final Answer:

   SELECT department, COUNT(*) FROM employees GROUP BY department; -> Option C

4. Quick Check:

   GROUP BY syntax: SELECT ... FROM ... GROUP BY column [OK]

Hint: GROUP BY follows FROM and lists columns to group by [OK]

Common Mistakes:

• Using ORDER BY instead of GROUP BY
• Using WHERE to group rows
• Using HAVING without GROUP BY

3. Given the table sales with columns region and amount, what is the output of this query?

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

medium

A. A list of regions with the total sales amount per region.

B. A list of all sales amounts without grouping.

C. An error because SUM() cannot be used with GROUP BY.

D. A list of regions sorted by amount.

Solution

1. Step 1: Understand GROUP BY with aggregate functions

   The query groups rows by 'region' and calculates the sum of 'amount' for each group.

2. Step 2: Analyze the output

   The result shows each region once with the total sales amount summed from all rows in that region.

3. Final Answer:

   A list of regions with the total sales amount per region. -> Option A

4. Quick Check:

   GROUP BY + SUM() = totals per group [OK]

Hint: GROUP BY with SUM() gives totals per group [OK]

Common Mistakes:

• Expecting all rows without grouping
• Thinking SUM() causes error with GROUP BY
• Confusing grouping with sorting

4. What is wrong with this query?

SELECT department, employee_name, COUNT(*) FROM employees GROUP BY department;

medium

A. You cannot select employee_name without grouping by it or using an aggregate function.

B. COUNT(*) cannot be used with GROUP BY.

C. GROUP BY must come before SELECT.

D. The query is correct and will run without errors.

Solution

1. Step 1: Check columns in SELECT with GROUP BY

   When using GROUP BY on 'department', all selected columns must be grouped or aggregated.

2. Step 2: Identify the error

   'employee_name' is neither grouped nor aggregated, causing a syntax error.

3. Final Answer:

   You cannot select employee_name without grouping by it or using an aggregate function. -> Option A

4. Quick Check:

   Non-grouped columns must be aggregated [OK]

Hint: All selected columns must be grouped or aggregated [OK]

Common Mistakes:

• Selecting non-grouped columns without aggregation
• Thinking COUNT(*) is invalid with GROUP BY
• Misplacing GROUP BY clause

5. You want to find departments with more than 5 employees. Which query correctly uses GROUP BY and HAVING to achieve this?

hard

A. SELECT department, COUNT(*) FROM employees GROUP BY department WHERE COUNT(*) > 5;

B. SELECT department, COUNT(*) FROM employees HAVING COUNT(*) > 5 GROUP BY department;

C. SELECT department, COUNT(*) FROM employees WHERE COUNT(*) > 5 GROUP BY department;

D. SELECT department, COUNT(*) FROM employees GROUP BY department HAVING COUNT(*) > 5;

Solution

1. Step 1: Understand HAVING clause usage

   HAVING filters groups after aggregation, so it must come after GROUP BY.

2. Step 2: Check query order and syntax

   SELECT department, COUNT(*) FROM employees GROUP BY department HAVING COUNT(*) > 5; correctly places HAVING after GROUP BY with condition COUNT(*) > 5. Other options misuse HAVING or WHERE clauses.

3. Final Answer:

   SELECT department, COUNT(*) FROM employees GROUP BY department HAVING COUNT(*) > 5; -> Option D

4. Quick Check:

   HAVING filters groups after GROUP BY [OK]

Hint: Use HAVING after GROUP BY to filter groups [OK]

Common Mistakes:

• Using WHERE to filter aggregated results
• Placing HAVING before GROUP BY
• Confusing WHERE and HAVING clauses