Aggregates with DISTINCT
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