COUNT

Returns the number of rows matching the query criteria.

SELECT COUNT(*) FROM Customers WHERE FirstName = 'jdoe1234'

COUNT(DISTINCT)

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

SELECT COUNT(DISTINCT FirstName) AS DistinctValues FROM Customers WHERE FirstName = 'jdoe1234'

AVG

Returns the average of the column values.

SELECT Id, AVG(Size) FROM Customers WHERE FirstName = 'jdoe1234'  GROUP BY Id

MIN

Returns the minimum column value.

SELECT MIN(Size), Id FROM Customers WHERE FirstName = 'jdoe1234' GROUP BY Id

MAX

Returns the maximum column value.

SELECT Id, MAX(Size) FROM Customers WHERE FirstName = 'jdoe1234' GROUP BY Id

SUM

Returns the total sum of the column values.

SELECT SUM(Size) FROM Customers WHERE FirstName = 'jdoe1234'