Aggregates with DISTINCT
From SQLZOO
Nobel database
COUNT DISTINCT
You can find the number of different values using COUNT with DISTINCT
In 1915 four prizes were awarded in three different subjects:
SELECT yr, subject, winner FROM nobel WHERE yr=1915 

COUNT(subject) gives 4; COUNT(DISTINCT subject) gives 3.
SELECT COUNT(subject), COUNT(DISTINCT subject) FROM nobel WHERE yr = 1915
SELECT COUNT(subject), COUNT(DISTINCT subject) FROM nobel WHERE yr = 1915