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

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this DISTINCT query?

Given a table Employees with a column Department containing values: 'Sales', 'HR', 'Sales', 'IT', 'HR', what will this query return?

SELECT DISTINCT Department FROM Employees;

SQL

SELECT DISTINCT Department FROM Employees;

A Sales, HR, Sales, IT, HR

B Sales, HR, IT

C Sales, IT

D HR, IT

💡 Hint

DISTINCT removes duplicate values and shows only unique ones.

The DISTINCT keyword returns only unique values from the Department column, so duplicates like 'Sales' and 'HR' appear once.

❓ query_result

intermediate

How many rows does this DISTINCT query return?

Consider a table Orders with a column CustomerID having values: 1, 2, 2, 3, 1, 4.
What is the number of rows returned by this query?

SELECT DISTINCT CustomerID FROM Orders;

SQL

SELECT DISTINCT CustomerID FROM Orders;

A 6

B 5

C 4

D 3

💡 Hint

Count unique CustomerID values only.

The unique CustomerIDs are 1, 2, 3, and 4, so the query returns 4 rows.

📝 Syntax

advanced

Which query correctly uses DISTINCT to get unique city names?

You want to get unique city names from the Customers table's City column. Which query is correct?

A SELECT DISTINCT City FROM Customers;

B SELECT City DISTINCT FROM Customers;

C SELECT City FROM Customers DISTINCT;

D SELECT City FROM DISTINCT Customers;

💡 Hint

DISTINCT comes right after SELECT and before column names.

The correct syntax places DISTINCT immediately after SELECT, before the column name.

❓ optimization

advanced

Which query is more efficient to get unique product categories?

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

A SELECT Category FROM Products GROUP BY Category;

B SELECT Category FROM Products;

C SELECT Category, COUNT(*) FROM Products;

D SELECT DISTINCT Category FROM Products;

💡 Hint

DISTINCT is designed to return unique values directly.

SELECT DISTINCT is optimized to return unique values without grouping overhead.

🧠 Conceptual

expert

What error occurs when using DISTINCT with multiple columns incorrectly?

Consider this query:

SELECT DISTINCT Name, Age FROM Employees WHERE Age > 30;

What happens if you try to use DISTINCT on only one column but select multiple columns without specifying DISTINCT properly?

A Returns unique rows based on all selected columns combined

B SyntaxError because DISTINCT must be applied to all selected columns

C Returns unique values only for the first column, ignoring others

D Runtime error due to ambiguous DISTINCT usage

💡 Hint

DISTINCT applies to the entire row, not just one column.

DISTINCT returns unique rows considering all selected columns together, not individually.

Practice

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

easy

A. It sorts the results in ascending order.

B. It returns only unique rows by removing duplicates.

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

D. It updates duplicate rows with new values.

Solution

1. Step 1: Understand the purpose of DISTINCT

   The DISTINCT keyword is used to filter out duplicate rows in the result set, so only unique rows remain.

2. Step 2: Compare with other options

   Sorting is done by ORDER BY, counting by COUNT(), and updating by UPDATE statement, none of which are related to DISTINCT.

3. Final Answer:

   It returns only unique rows by removing duplicates. -> Option B

4. Quick Check:

   DISTINCT = unique rows [OK]

Hint: DISTINCT removes duplicates, showing only unique rows [OK]

Common Mistakes:

• Confusing DISTINCT with ORDER BY
• Thinking DISTINCT counts rows
• Assuming DISTINCT updates 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 UNIQUE city FROM Customers;

C. SELECT city UNIQUE FROM Customers;

D. SELECT city FROM Customers DISTINCT;

Solution

1. Step 1: Recall correct DISTINCT syntax

   The correct syntax to get unique values is SELECT DISTINCT column_name FROM table_name.

2. Step 2: Check each option

   SELECT DISTINCT city FROM Customers; matches the correct syntax. The other options use incorrect keyword placement or invalid keywords.

3. Final Answer:

   SELECT DISTINCT city FROM Customers; -> Option A

4. Quick Check:

   Correct syntax uses DISTINCT after SELECT [OK]

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

Common Mistakes:

• Using UNIQUE instead of DISTINCT
• Placing DISTINCT after FROM
• Writing keywords in wrong order

3. Given the table Orders with a column Product containing values: ['Pen', 'Pen', 'Book', 'Notebook', 'Book'], what will the query SELECT DISTINCT Product FROM Orders; return?

medium

A. Pen, Book, Notebook

B. Pen, Book

C. Pen, Pen, Book, Notebook, Book

D. Notebook

Solution

1. Step 1: Identify duplicates in Product column

   The values are Pen (twice), Book (twice), and Notebook (once).

2. Step 2: Apply DISTINCT to remove duplicates

   DISTINCT returns each unique value once: Pen, Book, Notebook.

3. Final Answer:

   Pen, Book, Notebook -> Option A

4. Quick Check:

   DISTINCT removes duplicates, leaving unique values [OK]

Hint: DISTINCT returns each unique value once [OK]

Common Mistakes:

• Listing duplicates as separate rows
• Missing some unique values
• Confusing DISTINCT with COUNT

4. What is wrong with this query to get unique customer names from Customers table?
SELECT DISTINCT name, age FROM Customers;
Assuming you want unique names only.

medium

A. The query needs GROUP BY instead of DISTINCT.

B. DISTINCT cannot be used with multiple columns.

C. DISTINCT should be replaced with UNIQUE.

D. DISTINCT applies to all selected columns, so duplicates in name may remain if age differs.

Solution

1. Step 1: Understand how DISTINCT works with multiple columns

   DISTINCT treats the combination of all selected columns as one unit, so rows are unique only if all columns differ.

2. Step 2: Analyze the effect on name uniqueness

   If the same name appears with different ages, both rows appear because the combination is unique, not just the name.

3. Final Answer:

   DISTINCT applies to all selected columns, so duplicates in name may remain if age differs. -> Option D

4. Quick Check:

   DISTINCT on multiple columns = unique row combos [OK]

Hint: DISTINCT on multiple columns checks combined uniqueness [OK]

Common Mistakes:

• Thinking DISTINCT applies to each column separately
• Using UNIQUE keyword instead of DISTINCT
• Confusing DISTINCT with GROUP BY

5. You have a table Sales with columns Region and SalesPerson. You want to find all unique salespersons regardless of region. Which query gives the correct result?

hard

A. SELECT SalesPerson FROM Sales GROUP BY Region;

B. SELECT DISTINCT Region, SalesPerson FROM Sales;

C. SELECT DISTINCT SalesPerson FROM Sales;

D. SELECT UNIQUE SalesPerson FROM Sales;

Solution

1. Step 1: Identify the goal

   You want unique salespersons regardless of region, so only SalesPerson column matters.

2. Step 2: Choose query that returns unique SalesPerson values

   SELECT DISTINCT SalesPerson FROM Sales returns unique salespersons. SELECT DISTINCT Region, SalesPerson FROM Sales; returns unique pairs of Region and SalesPerson, which may repeat salespersons. SELECT SalesPerson FROM Sales GROUP BY Region; groups by Region, not SalesPerson, so it won't give unique salespersons. SELECT UNIQUE SalesPerson FROM Sales; uses UNIQUE which is invalid syntax.

3. Final Answer:

   SELECT DISTINCT SalesPerson FROM Sales; -> Option C

4. Quick Check:

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

Hint: DISTINCT only on needed column for unique values [OK]

Common Mistakes:

• Selecting multiple columns with DISTINCT causing duplicates
• Using GROUP BY on wrong column
• Using UNIQUE keyword instead of DISTINCT