COUNT operates on entire rows. The others operate on specific columns.
Figure shows examples of aggregate queries.
NULLs are skipped in column aggregates, but are processed by COUNT.
Aggregates can be combined with the WHERE clause to produce more complex
results. The query SELECT AVG(age) FROM friends WHERE age >= 21 computes
the average age of people age 21 or older. This prevents Dick Gleason
from being included in the average computation because he is younger than 21.
The column label defaults to the name of the aggregate. You can use AS
to change it, as shown in section .