A problem on running a sql script in DBVisualizer

Sorry for this question is somewhat off the topic of this mailing list. I use the DBVisualizer to run a sql script. I have 
somethink like:

drop table a;

create table a(. . .
);

drop table b;

create table b (. . .
)
inherits a;

I get an error:  Relation "b" inherits from "a"

It is fine when I run the sql statements line by line.

How to overcome this problem?

Thanks in advance.

Vernon