ij's SHOW and DESCRIBE commands use DatabaseMetaData in order to be portable to other databases than Derby, and in many cases they work fine with other databases. However, the variants of the commands that take a table name or a schema name assume that unquoted identifiers are converted to and stored in upper case internally. This is not true for all databases, and since these commands don't accept quoted identifiers, there's currently no way to access tables/schemas that are not all upper case.
One possible fix is to make the identifier() method in ij.jj use the DatabaseMetaData methods storesLowerCaseIdentifiers(), storesMixedCaseIdentifiers() and storesUpperCaseIdentifiers() to decide whether it should convert the identifier to lower case, keep it unchanged, or convert it to upper case. Currently, that method always converts the identifiers to upper case.