COUNT

Returns the number of rows matching the query criteria.

SELECT COUNT(*) FROM Customers WHERE Country = 'US'

COUNT(DISTINCT)

Returns the number of distinct, non-null field values matching the query criteria.

SELECT COUNT(DISTINCT City) AS DistinctValues FROM Customers WHERE Country = 'US'

AVG

Returns the average of the column values.

SELECT CompanyName, AVG(Balance) FROM Customers WHERE Country = 'US'  GROUP BY CompanyName

MIN

Returns the minimum column value.

SELECT MIN(Balance), CompanyName FROM Customers WHERE Country = 'US' GROUP BY CompanyName

MAX

Returns the maximum column value.

SELECT CompanyName, MAX(Balance) FROM Customers WHERE Country = 'US' GROUP BY CompanyName

SUM

Returns the total sum of the column values.

SELECT SUM(Balance) FROM Customers WHERE Country = 'US'