WHERE parent IN (0,-1) - Mailing list pgsql-sql

Hi,

I'm pretty new to Postgres, so please excuse my ignorance.

I have a sql script that uses WHERE parent IN (0,-1), this works fine with
MySQL, Access, & MS-SQL Server, unfortunately Postgres does not like this
statement, I can get around the problem (I think) by using WHERE parent IN
(0,'-1'). 

Is this valid? 

Is there another way to do this that works with Postgres and the others?
(They dont like the ' around -1)

Thanks,

Drew