[OT}Difference JDBC connection for AS400 and Postgres

HiWhen i have to create a JDBC connection for AS400, i can set the AS400
library name in my JDBC url like
jdbc:as400:myas400/mycollect;naming=system;
and then write a query select * from mytable

Where mycollect is my database, so if i change the url
to jdbc:as400:myas400/mycollect1, i can run the same query with out having
to change SQL query

but in postgres or oracle, i have to write my query as
select * from mycollect.mytable

So if i want to get data from other database, i will have to modify call the
queries.

Is this correct, or is there some other way of specifying database name in
JDBC connection