COUNT

Returns the number of rows matching the query criteria.

SELECT COUNT(*) FROM Invoices WHERE Supplier = 'Bob'

COUNT(DISTINCT)

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

SELECT COUNT(DISTINCT InvoiceId) AS DistinctValues FROM Invoices WHERE Supplier = 'Bob'

AVG

Returns the average of the column values.

SELECT InvoiceId, AVG(AnnualRevenue) FROM Invoices WHERE Supplier = 'Bob'  GROUP BY InvoiceId

MIN

Returns the minimum column value.

SELECT MIN(AnnualRevenue), InvoiceId FROM Invoices WHERE Supplier = 'Bob' GROUP BY InvoiceId

MAX

Returns the maximum column value.

SELECT InvoiceId, MAX(AnnualRevenue) FROM Invoices WHERE Supplier = 'Bob' GROUP BY InvoiceId

SUM

Returns the total sum of the column values.

SELECT SUM(AnnualRevenue) FROM Invoices WHERE Supplier = 'Bob'