create table t1(c1 char(5),c2 integer);
create table t2(c1 char(5),c2 integer);

insert into t1 values('A',100),('B',200);
insert into t2(c1) values('A'),('B'),('C');

select * from t1;

C1    C2        
----- -----------
A             100
B             200

select * from t2;

C1    C2        
----- -----------
A               -
B               -
C               -

update t2 set c2=(select c2 from t1 where t1.c1=t2.c1);

select * from t2;

C1    C2        
----- -----------
A             100
B             200
C               -

insert into t1 values('A',200);

update t2 set c2=(select c2 from t1 where t1.c1=t2.c1);

DB21034E  명령이 유효한 명령행 처리기 명령이 아니므로 SQL문으로 처리되었습니다.
SQL 처리 중에 이 명령은 다음을 리턴했습니다.
SQL0811N  스칼라 fullselect, SELECT INTO문 또는 VALUES INTO문의 결과가 두 행
이상입니다.  SQLSTATE=21000

update t2 set c2=(select max(c2) from t1 where t1.c1=t2.c1);

select * from t2;

C1    C2        
----- -----------
A             200
B             200
C               -

update t2 set c2=50 where c1 = 'C';

select * from t2;

C1    C2        
----- -----------
A             200
B             200
C              50

update t2 set c2=(select max(c2) from t1 where t1.c1=t2.c1);

select * from t2;

C1    C2        
----- -----------
A             200
B             200
C               -

update t2 set c2=50 where c1 = 'C'

update t2 set c2=(select max(c2) from t1 where t1.c1=t2.c1) where t2.c1 in (select t1.c1 from t1)

select * from t2;

C1    C2        
----- -----------
A             200
B             200
C              50

도님 감사합니다...  ^^*