The SELECT DISTINCT statement returns the distinct (different) values of a column.
Unlike SELECT that returns all values of a column.
You can run the bellow queries and compare the result.
-- return distinct of company names(which is unique values)
SELECT DISTINCT company
FROM products
ORDER BY company;
-- +---------------------------------+
-- | company |
-- |---------------------------------|
-- | Adkins, Short and Maxwell |
-- | Alexander Inc |
-- | Allen-Jones |
-- | Anderson-Santiago |
-- | Andrews Group |
-- | Austin, Brown and Marshall |
-- | Baker and Sons |
-- ...
-- without distinct, which retuns company names with duplicate values
SELECT company
FROM products
ORDER BY company;
-- +---------------------------------+
-- | company |
-- |---------------------------------|
-- | Adkins, Short and Maxwell |
-- | Adkins, Short and Maxwell |
-- | Alexander Inc |
-- | Allen-Jones |
-- | Allen-Jones |
-- | Anderson-Santiago |
-- | Andrews Group |
-- | Andrews Group |
-- ...
-- use count and get the number of companies
SELECT COUNT(DISTINCT company) FROM products;
-- +-------+
-- | count |
-- |-------|
-- | 122 |
-- +-------+
