Quick single scan solution

😎

USE tempdb;

GO

SET NOCOUNT ON;

WITH XSAMPLE(book, position) AS

(SELECT * FROM (VALUES

('A', 500)

,('B',-500)

,('C',-500)

) AS X(book, position))

,BALANCE_STUFF AS

(

SELECT

XS.book

,XS.position

,SUM(XS.position) OVER

(

PARTITION BY (SELECT NULL)

ORDER BY XS.book

ROWS BETWEEN UNBOUNDED PRECEDING

AND CURRENT ROW

) AS RT_BALANCE

FROM XSAMPLE XS

)

SELECT

BS.book

,BS.position

FROM BALANCE_STUFF BS

WHERE BS.RT_BALANCE <> 0;