Row duplication - SQL limitation

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-- Here is an example of an SQL limitation.

-- Joining more than one table on the same columns that contains more than on

-- record per join criteria causes row duplication to occur.

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SELECT  t1.InvestorID , t2.Period , t3.Period

SELECT NumRecords = COUNT(*)

FROM    dbo.InvestorBase t1 WITH (NOLOCK)

        LEFT JOIN dbo.InvestorDetail1 t2 WITH (NOLOCK)
            ON t1.InvestorID      = t2.InvestorID 

            AND t1.Period         = t2.Period

        LEFT JOIN dbo.InvestorDetail2 t3 WITH (NOLOCK) 
            ON t1.InvestorID      = t3.InvestorID 

            AND t1.Period         = t3.Period