DISTINCT

Some questions have repeated answers. Ask "which countries do our customers live in?" and India comes back three times:

SELECT country FROM customers;

SELECT DISTINCT

DISTINCT removes duplicate rows from the result:

SELECT DISTINCT country
FROM customers
ORDER BY country;

Ten rows become eight, with each country once. The table itself is unchanged: DISTINCT only affects what the query returns.

DISTINCT on several columns

With more than one column, a row is a duplicate only if every column matches. The two Engineers collapse into one row, but an Engineer and a Lead Engineer stay separate:

SELECT DISTINCT department, title
FROM employees
ORDER BY department, title;

Counting distinct values

Put DISTINCT inside COUNT to count unique values:

SELECT
  COUNT(*)                    AS orders,
  COUNT(DISTINCT customer_id) AS customers_who_ordered,
  COUNT(DISTINCT status)      AS statuses
FROM orders;

Twelve orders came from nine different customers, in four different statuses.

Exploring a column

DISTINCT is a quick way to see what values a column holds before you filter on it:

SELECT DISTINCT status FROM orders ORDER BY status;