表T1和表T2，表T2的F23字段用表T1的F3字段更新，条件是T1.F2=T2.F22并且只有T1.F2和T2.F22是一一对应，T1只有一条记录和T2对应，例子如下：
create table t1(f1 int,f2 varchar(10),f3 varchar(10));
create table t2(f21 int,f22 varchar(10),f23 varchar(10)); insert into t1 values(1,'111','abc'); insert into t1 values(2,'abc'); insert into t1 values(3,'222','ddd'); insert into t2 values(1,null); insert into t2 values(2,null); update t2 set t2.f23 = t1.f3 from t1,t2 where t1.f2=t2.f22 and exists(select t3.f2,COUNT(*) from t1 as t3 where t3.f2=t1.f2 group by t3.f2 having COUNT(*) =1 ); select * from t1; select * from t2;