In case of PostgreSQL it is documented here.

With JDBC, a database is represented by a URL (Uniform Resource Locator). With PostgreSQL™, this takes one of the following forms:

• jdbc:postgresql:database
• jdbc:postgresql://host/database
• jdbc:postgresql://host:port/database

In case of MySQL it is documented here.

The general format for a JDBC URL for connecting to a MySQL server is as follows, with items in square brackets ([ ]) being optional:

jdbc:mysql://[host1][:port1][,[host2][:port2]]...[/[database]] » [?propertyName1=propertyValue1[&propertyName2=propertyValue2]...]

In case of Oracle it is documented here.

There are 2 URL syntax, old syntax which will only work with SID and the new one with Oracle service name.

Old syntax jdbc:oracle:thin:@[HOST][:PORT]:SID

New syntax jdbc:oracle:thin:@//[HOST][:PORT]/SERVICE