update tableB set A4='y'
from tableA a
where a.A1= tableB .A1 and a.A2= tableB .A2 and a.A3= tableB .A3
from tableA a
where a.A1= tableB .A1 and a.A2= tableB .A2 and a.A3= tableB .A3
调试欢乐多
from tableA a
join tableB b on a.A1=b.A1 and a.A2=b.A2 and a.A3=b.A3