UPDATE A
SET A.COL1=(SELECT B.COL1 FROM B WHERE A.KEY=B.KEY)
SQL SERVER :
UPDATE A
SET A.COL1=B.COL WHERE A.KEY=B.KEY1
FROM A,B
UPDATE A
SET A.COL1=(SELECT B.COL1 FROM B WHERE A.KEY=B.KEY)
SQL SERVER :
UPDATE A
SET A.COL1=B.COL WHERE A.KEY=B.KEY1
FROM A,B