1.update tableB set c=(select top 1 c from tableA where tableB.a=tableA.a and tableB.b=tableA.b)
2.update tableB set c= tableA.c
from tableB,tableA where tableB.a=tableA.a and tableB.b=tableA.b
update B set c = A.c from A,B where A.a = B.a and A.b = B.b
update A set A.c=B.c
from A
inner join B on A.a=B.a and A.b=B.b