How to write a constraint which need to check other table?

[纪晓曦 <sheepjxx@xxxxxxxxx>]

create table a(
         name varchar(32);
);

create talbe b(
          name1 varchar(32);
          name2 varchar(32);
);


How to write a constraint to check name1, name2 in the table a without change table defination?


ALTER TABLE b ADD CHECK(  ??? );