SQL Select Distinct

The SELECT DISTINCT statement is used to remove duplicate rows from your query results and return only unique values.

When a database table contains repetitive information (like a list of customers where many live in the same city), DISTINCT filters out the duplicates so you see each value exactly once.

Basic Syntax for a Single Column

To find all unique values within one specific column, place DISTINCT immediately after SELECT.

SELECT DISTINCT column_name
FROM table_name;

Real-World Example:

Imagine you have an employees table, and multiple people work in the same department.

-- This returns a clean list of unique departments, hiding duplicates

SELECT DISTINCT department
FROM employees;

Using DISTINCT with Multiple Columns

When you list multiple columns after DISTINCT, SQL evaluates the combination of those columns.

A row is only hidden if the exact combination of all selected columns repeats.

-- Returns unique combinations of department AND job_title

SELECT DISTINCT department, job_title
FROM employees;

If "Sales" has three "Managers" and two "Associates", the output will display "Sales | Manager" once and "Sales | Associate" once.

Counting Unique Values (COUNT)

You can combine DISTINCT with the COUNT() function to find the exact number of unique items in a column.

-- Calculates the total number of unique cities your customers live in

SELECT COUNT(DISTINCT city) AS unique_city_count
FROM customers;

Note: COUNT(DISTINCT column) automatically ignores NULL values in most database platforms).

Key Rules to Remember

• Position Matters: DISTINCT must always be placed directly after SELECT. Writing SELECT column, DISTINCT another_column will result in a syntax error.
• Performance Impact: To find unique values, the database engine must sort and compare your data. Avoid using DISTINCT on millions of rows unless it is absolutely necessary, as it can slow down query speeds.