DISTINCT for unique values in MySQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this DISTINCT query?

Consider a table Employees with a column Department containing values: 'Sales', 'HR', 'Sales', 'IT', 'HR'. What will this query return?

SELECT DISTINCT Department FROM Employees;

MySQL

SELECT DISTINCT Department FROM Employees;

A Sales, HR, Sales, IT, HR

B Sales, HR, IT

C Sales, IT

D HR, IT

💡 Hint

DISTINCT removes repeated values and shows each unique value once.

The DISTINCT keyword returns only unique values from the Department column, so duplicates like repeated 'Sales' or 'HR' are shown once.

🧠 Conceptual

intermediate

Which statement about DISTINCT is true?

Choose the correct statement about the DISTINCT keyword in SQL.

A DISTINCT changes the order of rows returned.

B DISTINCT only works on numeric columns.

C DISTINCT removes duplicate rows based on all selected columns.

D DISTINCT duplicates all rows in the result.

💡 Hint

Think about what DISTINCT does to repeated data.

DISTINCT removes duplicate rows considering all columns in the SELECT list, not just one column or only numeric data.

📝 Syntax

advanced

Which query correctly uses DISTINCT to get unique city names?

Given a table Customers with a column City, which query correctly returns unique city names?

A SELECT UNIQUE City FROM Customers;

B SELECT City DISTINCT FROM Customers;

C SELECT City FROM Customers DISTINCT;

D SELECT DISTINCT City FROM Customers;

💡 Hint

DISTINCT comes right after SELECT.

The correct syntax is SELECT DISTINCT column FROM table. Other options have wrong keyword order or unsupported keywords.

❓ optimization

advanced

Which query is more efficient to get unique product categories?

You want to list unique product categories from a large Products table. Which query is more efficient?

A SELECT DISTINCT Category FROM Products;

B SELECT Category FROM Products GROUP BY Category;

C SELECT Category FROM Products;

D SELECT ALL Category FROM Products;

💡 Hint

DISTINCT and GROUP BY can both get unique values, but one is simpler and often faster.

SELECT DISTINCT is simpler and usually more efficient than GROUP BY when only unique values are needed without aggregation.

🔧 Debug

expert

Why does this query return duplicates despite DISTINCT?

Given a table Orders with columns OrderID, CustomerID, and OrderDate, why does this query return duplicate CustomerIDs?

SELECT DISTINCT CustomerID, OrderDate FROM Orders;

A DISTINCT applies to all selected columns, so different OrderDates cause duplicates.

B DISTINCT only works on the first column listed.

C DISTINCT is ignored when multiple columns are selected.

D DISTINCT removes duplicates only if OrderID is included.

💡 Hint

DISTINCT looks at the whole row, not just one column.

DISTINCT considers all columns in the SELECT list. If CustomerID repeats with different OrderDates, rows are unique and all show.

Practice

1. What does the DISTINCT keyword do in a MySQL query?

easy

A. It sorts the results in ascending order.

B. It updates values in a column.

C. It counts the number of rows in a table.

D. It returns only unique values by removing duplicates.

Solution

1. Step 1: Understand the purpose of DISTINCT

   The DISTINCT keyword is used to filter out duplicate rows in the result set.

2. Step 2: Compare with other options

   Sorting is done by ORDER BY, counting by COUNT(), and updating by UPDATE statement, so these are incorrect.

3. Final Answer:

   It returns only unique values by removing duplicates. -> Option D

4. Quick Check:

   DISTINCT = unique values [OK]

Hint: DISTINCT removes duplicates, giving unique results only [OK]

Common Mistakes:

• Confusing DISTINCT with ORDER BY
• Thinking DISTINCT counts rows
• Using DISTINCT to update data

2. Which of the following is the correct syntax to select unique city names from a table called customers?

easy

A. SELECT DISTINCT city FROM customers;

B. SELECT city UNIQUE FROM customers;

C. SELECT UNIQUE city FROM customers;

D. SELECT city FROM customers DISTINCT;

Solution

1. Step 1: Recall correct syntax for unique selection

   MySQL uses DISTINCT keyword immediately after SELECT to get unique values.

2. Step 2: Check each option

   SELECT DISTINCT city FROM customers; uses correct syntax: SELECT DISTINCT city FROM customers; others are invalid syntax.

3. Final Answer:

   SELECT DISTINCT city FROM customers; -> Option A

4. Quick Check:

   Syntax for unique values = SELECT DISTINCT [OK]

Hint: Use SELECT DISTINCT column FROM table for unique values [OK]

Common Mistakes:

• Using UNIQUE instead of DISTINCT in SELECT
• Placing DISTINCT after FROM clause
• Misplacing keywords causing syntax errors

3. Given the table orders with a column product_id containing values: [1, 2, 2, 3, 1, 4], what will the query SELECT DISTINCT product_id FROM orders; return?

medium

A. [1, 2, 3, 4]

B. [1, 2, 2, 3, 1, 4]

C. [2, 3, 4]

D. [1, 3, 4]

Solution

1. Step 1: Identify duplicates in product_id

   The values are 1, 2, 2, 3, 1, 4. Duplicates are 1 and 2 repeated.

2. Step 2: Apply DISTINCT to remove duplicates

   Removing duplicates leaves unique values: 1, 2, 3, 4.

3. Final Answer:

   [1, 2, 3, 4] -> Option A

4. Quick Check:

   DISTINCT removes duplicates = unique list [OK]

Hint: DISTINCT returns only one of each repeated value [OK]

Common Mistakes:

• Expecting all original values including duplicates
• Confusing DISTINCT with sorting
• Ignoring repeated values in output

4. What is wrong with this query if you want unique customer names from the customers table?
SELECT DISTINCT name, age FROM customers;
Assuming some customers share the same name but have different ages.

medium

A. It will cause a syntax error.

B. It will return unique names only, ignoring age.

C. It will return unique combinations of name and age, not just names.

D. It will return duplicate names.

Solution

1. Step 1: Understand DISTINCT with multiple columns

   DISTINCT applies to the entire row of selected columns, so it returns unique pairs of name and age.

2. Step 2: Analyze the effect on names

   If customers share the same name but different ages, each pair is unique, so duplicates in names alone are not removed.

3. Final Answer:

   It will return unique combinations of name and age, not just names. -> Option C

4. Quick Check:

   DISTINCT on multiple columns = unique rows [OK]

Hint: DISTINCT on multiple columns checks all columns together [OK]

Common Mistakes:

• Thinking DISTINCT applies to each column separately
• Expecting only unique names when selecting multiple columns
• Assuming syntax error for multiple columns with DISTINCT

5. You have a table sales with columns region and salesperson. Some salespersons work in multiple regions. Which query will list all unique salespersons regardless of region?

hard

A. SELECT DISTINCT region, salesperson FROM sales;

B. SELECT DISTINCT salesperson FROM sales;

C. SELECT salesperson FROM sales GROUP BY region;

D. SELECT salesperson FROM sales WHERE DISTINCT region;

Solution

1. Step 1: Identify the goal

   We want unique salespersons only, ignoring regions.

2. Step 2: Analyze each query

   SELECT DISTINCT region, salesperson FROM sales; returns unique pairs of region and salesperson, so duplicates by salesperson remain if regions differ. SELECT DISTINCT salesperson FROM sales; returns unique salespersons only. SELECT salesperson FROM sales GROUP BY region; groups by region, not salesperson, so duplicates remain. SELECT salesperson FROM sales WHERE DISTINCT region; is invalid syntax.

3. Final Answer:

   SELECT DISTINCT salesperson FROM sales; -> Option B

4. Quick Check:

   Distinct on salesperson column only = unique salespersons [OK]

Hint: Use DISTINCT on the single column you want unique values for [OK]

Common Mistakes:

• Selecting multiple columns with DISTINCT when only one is needed
• Using GROUP BY on wrong column
• Writing invalid WHERE DISTINCT syntax