Running totals. Traditionally done with slow set-based code, with a (relatively!) fast cursor, or with some dangerous unsupported side effect of a language feature.

In SQL Server 2012, very simple:

SELECT SalesPersonName,

SaleDate,

Amount,

SUM(Amount) OVER (PARTITION BY SalesPersonName

ORDER BY SaleDate

ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS RunningTotal

FROM YourTable

ORDER BY SalesPersonName, SaleDate;