Unique values with DISTINCT

Sometimes a column contains repeated values and you only care about the list of distinct values. The keyword DISTINCT placed right after SELECT removes duplicates from the result:

SELECT DISTINCT <colonne>
FROM   <tabella>
[WHERE <condizione>];

Uniqueness is evaluated over the combination of all listed columns. If you list two columns, two rows are considered equal only when both values match.

Examples

-- Le città in cui abbiamo almeno un dipartimento:
SELECT DISTINCT city FROM departments;

-- Le coppie (dipartimento, città) presenti (qui ognuna unica):
SELECT DISTINCT department_id, hired_on
FROM employees;

Try it

Review exercise