SQL 用一个表的数据更新另一张表

用表B的数据（B1列,B2）更新表A的A1,A2列

SQL Server:

update A SET A.A1 = b.B1,A.A2=B.A2 FROM A ,B WHERE  A.ID1 = B.ID1 and A.ID2 = B.ID2;

Access:

update A, B  set A.A1 = B.B1,A.A2=B.B2 where A.ID1 = B.ID1 and A.ID2 = B.ID2;

--或

update A INNER JOIN B ON A.ID1 = B.ID1 AND A.ID2= B.ID2 SET A.A1 = B.B1,A.A2=B.B2;

通过一条ＳＱＬ语句一次更新多条数据．ＳＱＬ语句写法略有不同，如下：

方法一:

Update T1
set dc=(select dc1 from t2 where T1.A=T2.A1 AND T1.B=T2.B1)
WHERE T1.AID in (
    SELECT AID FROM T1
    INNER JOIN T2 ON t1.A=T2.A1 AND T1.B=T2.B1
)

注：在我标我红色的in地方如果把'in'换成'='将出错，但在下面的Ｂ种写法里就可以换成'=',变成Ｃ方法写法.

方法二：

Update T1
set dc=(select dc1 from t2 where T1.A=T2.A1 AND T1.B=T2.B1)
WHERE T1.AID in (
    SELECT T1.AID FROM T2
    WHERE t1.A=T2.A1 AND T1.B=T2.B1
)

方法三：

Update T1
set dc=(select dc1 from t2 where T1.A=T2.A1 AND T1.B=T2.B1)
WHERE T1.AID = (
    SELECT T1.AID FROM T2
    WHERE t1.A=T2.A1 AND T1.B=T2.B1
)

方法四：

Update T1
set dc=(select dc1 from T2 where T1.A=T2.A1 AND T1.B=T2.B1)
WHERE T1.A = (
    SELECT T1.A FROM T2
    WHERE T1.A=T2.A1 AND T1.B=T2.B1
)

方法五：

Update T1
set dc=(select dc1 from T2 where T1.A=T2.A1 AND T1.B=T2.B1)
WHERE T1.A = (
    SELECT T2.A1 FROM T2
    WHERE T1.A=T2.A1 AND T1.B=T2.B1
)

方法六:　

Update T1
set dc=(select dc1 from T2 where T1.A=T2.A1 AND T1.B=T2.B1)
WHERE T1.A in (
    SELECT T2.A1 FROM T2
    WHERE T1.A=T2.A1 AND T1.B=T2.B1
)

注：如果在上面方法改写成如下的写法将会出错。

Update T1 
set dc=(select dc1 from T2 where T1.A=T2.A1 AND T1.B=T2.B1)
WHERE T1.A in (
    SELECT T1.A FROM T1 
    INNER JOIN T2 ON t1.A=T2.A1 AND T1.B=T2.B1
)

此种方法将会更新掉所有符合T1.A in ()这个条件的数据,所以是不正确的, 所以不能这样写。

方法七：

Update T1
set dc=(select dc1 from T2 where T1.A=T2.A1 AND T1.B=T2.B1 AND T1.DC=T2.DC1)
FROM T1, T2
WHERE  T1.A=T2.A1 AND T1.B=T2.B1

方法八：

Update T1 
set T1.dc=T2.dc1
FROM T1
INNER JOIN T2 ON T1.A=T2.A1 AND T1.B=T2.B1
WHERE t1.A=T2.A1 AND T1.B=T2.B1

方法九：

Update T1
set T1.dc=T2.dc1
FROM T1, T2
WHERE  T1.A=T2.A1 AND T1.B=T2.B1

请特别注意以上方法中我用红色标记上的代码写法。

总节：虽然是一条简单的更新语名但也有多种写法，且每种写法各有优缺点。

大家是否还有更简单的写法或者更容易理解的写法？如果有请提出来一起讨论，谢谢！

说明测试环境：以上代码在ＭＳＳＱＬ2005中验证是可行的．

　　注：此处我用灰色标记的部分可以去掉。