Another old habit to break if you are using SQL2008 or greater is to avoid GROUP BY when all you really need is an aggregated column value.

SELECT

A.Portfolio

,B.PortfolioID

,SUM(B.MarketValue) AS SumOfMV

,COUNT(B.SecID) AS [# of Securities]

FROM

ERTutAccounts A

INNER JOIN ERTutPositions B

ON A.PortfolioID = B.PortfolioID

GROUP BY

ERTutAccounts.Portfolio

,ERTutPositions.PortfolioID;

SELECT

A.Portfolio

,B.PortfolioID

,SUM(B.MarketValue) OVER (PARTITION BY A.Portfolio, B.PortfolioID) AS SumOfMV

,COUNT(B.SecID) OVER (PARTITION BY A.Portfolio, B.PortfolioID) AS [# of Securities]

FROM

ERTutAccounts A

INNER JOIN ERTutPositions B

ON A.PortfolioID = B.PortfolioID

You can still use GROUP BY of course if you need to group the portfolios themselves but just to do sums or counts (or min or max, etc) the OVER() clause makes things a lot easier.