DISTINCT for unique values in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the SQL keyword DISTINCT do?

It removes duplicate rows from the result set, showing only unique values for the selected columns.

beginner

How would you select unique city names from a table called Customers?

Use SELECT DISTINCT city FROM Customers; to get only unique city names without duplicates.

intermediate

Can DISTINCT be used with multiple columns?

Yes, it returns unique combinations of the specified columns. For example, SELECT DISTINCT city, country FROM Customers; shows unique city-country pairs.

beginner

What happens if you use DISTINCT on a column with all unique values?

The result will be the same as a normal SELECT because all values are already unique.

intermediate

Is DISTINCT applied before or after sorting in a query?

DISTINCT is applied before sorting. The database first removes duplicates, then sorts the results if ORDER BY is used.

What does the SQL keyword DISTINCT do?

A Sorts the result in ascending order

B Removes duplicate rows from the result

C Limits the number of rows returned

D Joins two tables together

DISTINCT removes duplicate rows, showing only unique values.

Which query returns unique countries from the table Customers?

A SELECT DISTINCT country FROM Customers;

B SELECT UNIQUE country FROM Customers;

C SELECT country FROM Customers;

D SELECT country WHERE DISTINCT FROM Customers;

SELECT DISTINCT country FROM Customers; returns unique country names.

If you use DISTINCT on two columns, what do you get?

A All rows without filtering

B Unique values from the second column only

C Unique values from the first column only

D Unique combinations of both columns

DISTINCT on multiple columns returns unique combinations of those columns.

What will happen if all values in a column are already unique and you use DISTINCT?

A An error will occur

B Duplicates will be removed

C The result will be the same as a normal SELECT

D The query will run slower but results change

If all values are unique, DISTINCT does not change the result.

When is DISTINCT applied in the order of query operations?

A Before ORDER BY

B Before WHERE

C After GROUP BY

D After ORDER BY

DISTINCT is applied before ORDER BY sorting.

Explain how the DISTINCT keyword works in SQL and give an example query.

Think about how you would find unique items in a list.

Describe what happens when DISTINCT is used with multiple columns in a SELECT statement.

Imagine filtering unique pairs instead of single values.