Oralce和DB2都支持的语法：
UPDATE A SET (A1, A2, A3) = (SELECT B1, B2, B3 FROM B WHERE A.ID = B.ID)
MS SQL Server不支持这样的语法，相对应的写法为：
UPDATE A SET A1 = B1, A2 = B2, A3 = B3 FROM A LEFT JOIN B ON A.ID = B.ID
个人感觉MS SQL Server的Update语法功能更为强大。MS SQL SERVER的写法：
UPDATE A SET A1 = B1, A2 = B2, A3 = B3 FROM A, B WHERE A.ID = B.ID
在Oracle和DB2中的写法就比较麻烦了，如下：
UPDATE A SET (A1, A2, A3) = (SELECT B1, B2, B3 FROM B WHERE A.ID = B.ID)
WHERE ID IN (SELECT B.ID FROM B WHERE A.ID = B.ID)
或者
update test_a a set (a.name,a.age)= (select b.name,b.age from test_b b where a.id = b.id)
where exists (select * from test_b c where c.id=a.id)
