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.