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.