Just Have a look at the following:

CREATE TABLE AA(PRODCODE CHAR(5),
RATE DECIMAL(20,2) NOT NULL DEFAULT 0,
LAST_REC CHAR(1) NOT NULL DEFAULT ”)

INSERT AA VALUES(‘P1′,25,’L')
INSERT AA VALUES(‘P1′,0,”)

SELECT * FROM AA

CREATE TABLE BB(PRODCODE CHAR(5),
RATE DECIMAL(20,2) NOT NULL DEFAULT 0)

INSERT BB VALUES(‘P1′,0)
INSERT BB VALUES(‘P2′,0)

UPDATE BB SET RATE=
ISNULL((SELECT TOP 1 1/RATE FROM AA WHERE AA.PRODCODE=BB.PRODCODE),0)
– It says (2 row(s) affected)
– If TOP would not have executed first then we naturally get
– a divide by zero error as in the next command
UPDATE BB SET RATE=
ISNULL((SELECT TOP 1 1/RATE FROM AA WHERE AA.PRODCODE=BB.PRODCODE ORDER BY RATE),0)
– Msg 8134, Level 16, State 1, Line 1
–Divide by zero error encountered.
–The statement has been terminated.

– Thus we may also conclude that the evaluation of Expressions and Column values in select statement is also restricted on TOP. Only TOP % or Number rows –column are retrieved or expressions are evaluated. Executing Order BY first than TOP would naturally lead to a divide by zero eror.