UPDATE FROM portability

Updating some rows in tab1 with corresponding values from tab2, pgsql style:

UPDATE tab1
SET value=T2.VALUE
FROM tab2 T2
WHERE T2.restr=1
AND tab1.key=T2.key <<<<<<

The same for MSSQL:

UPDATE tab1
SET value=T2.VALUE
FROM tab1 T1
JOIN tab2 T2 ON T1.key=T2.Key <<<<<<
WHERE T2.restr=1

I'm looking for a portable query style, without using a subquery in the
SET clause (which could make things quite slow)
Any hints?

Regards,
Andreas