;WITH RepClientOrder
AS
(
	SELECT CompanyId, ClientId, EffectiveDate, SalesRep
		,ROW_NUMBER() OVER (PARTITION BY CompanyId, ClientID ORDER BY EffectiveDate) AS RowNum
	FROM ClientSalesReps
)
, RepClient
AS
(
	SELECT R1.CompanyId, R1.ClientId, R1.SalesRep
		,R1.EffectiveDate AS StartDate
		,COALESCE(R2.EffectiveDate, CAST('99991231' AS datetime)) AS EndDate
	FROM RepClientOrder R1
		LEFT JOIN RepClientOrder R2
			ON R1.CompanyId = R2.CompanyId
				AND R1.ClientId = R2.ClientId
				AND R1.RowNum = R2.RowNum - 1
)
-- select * from RepClientOrder order by CompanyId, ClientId, RowNum
-- select * from RepClient order by CompanyId, ClientId, StartDate
SELECT *
FROM RevenuePayers RP
	JOIN AccessionIndex AI
		ON RP.CompanyId = AI.CompanyId
			AND RP.ClientId = AI.ClientId
			AND RP.Accession = AI.Accession
	JOIN RepClient RC
		ON RP.CompanyId = RC.CompanyId
			AND RP.ClientId = RC.ClientId
			AND AI.DOS >= RC.StartDate
			AND AI.DOS < RC.EndDate
--etc