COUNT

Returns the number of rows matching the query criteria.

SELECT COUNT(*) FROM Orders WHERE TransactionID = '123'

COUNT(DISTINCT)

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

SELECT COUNT(DISTINCT TransactionID) AS DistinctValues FROM Orders WHERE TransactionID = '123'

AVG

Returns the average of the column values.

SELECT UserID, AVG(SalesTax) FROM Orders WHERE TransactionID = '123'  GROUP BY UserID

MIN

Returns the minimum column value.

SELECT MIN(SalesTax), UserID FROM Orders WHERE TransactionID = '123' GROUP BY UserID

MAX

Returns the maximum column value.

SELECT UserID, MAX(SalesTax) FROM Orders WHERE TransactionID = '123' GROUP BY UserID

SUM

Returns the total sum of the column values.

SELECT SUM(SalesTax) FROM Orders WHERE TransactionID = '123'