Later oracle database releases can access earlier

• Later Oracle Database releases can access earlier Oracle Database releases.
However, this access is only for upgrades. For example, Oracle Database 12 c
release 2 can access an Oracle Database 11 g release 2 (11.2.0.4) database if the
11.2.0.4 database is started up in upgrade mode.
• Oracle Database Client can be installed in the same Oracle Database home if both
products are at the same release level. For example, you can install Oracle
Database Client 12.2.0.1 into an existing Oracle Database 12.2.0.1 home but you
cannot install Oracle Database Client 12.2.0.1 into an existing Oracle Database
12.1.0.2 home. If you apply a patch set before installing the client, then you must
apply the patch set again.
• Structured organization of directories and files, and consistent naming for
database files simplify database administration.
• Login home directories are not at risk when database administrators add, move,
or delete Oracle home directories.
• You can test software upgrades in an Oracle home in a separate directory from the
Oracle home where your production database is located.
About Multiple Oracle Homes Support
C-2 Oracle Database Installation Guide

C.3 About the Oracle Inventory Directory and Installation
The directory that you designate as the Oracle Inventory directory (oraInventory)
stores an inventory of all software installed on the system.
All Oracle software installation owners on a server are granted the OINSTALL
privileges to read and write to this directory. If you have previous Oracle software
installations on a server, then additional Oracle software installations detect this
directory from the /etc/oraInst.loc file, and continue to use that Oracle
Inventory. Ensure that the group designated as the OINSTALL group is available as a
primary group for all planned Oracle software installation owners.
If you are installing Oracle software for the first time, then OUI creates an Oracle base
and central inventory, and creates an Oracle inventory using information in the
following priority:
• In the path indicated in the ORACLE_BASE environment variable set for the
installation owner user account
• In an Optimal Flexible Architecture (OFA) path (u[01–99] /app/ owner where
owner is the name of the user account running the installation), and that user
account has permissions to write to that path
• In the user home directory, in the path /app/ owner , where owner is the name of
the user account running the installation
For example:
If you are performing an Oracle Database installation, and you set ORACLE_BASE for
user oracle to the path /u01/app/oracle before installation, and grant 755
permissions to oracle for that path, then Oracle Universal Installer creates the Oracle
Inventory directory one level above the ORACLE_BASE in the path
ORACLE_BASE/../oraInventory , so the Oracle Inventory path is /u01/app/
oraInventory . Oracle Universal Installer installs the software in the ORACLE_BASE
path. If you are performing an Oracle Grid Infrastructure for a Cluster installation,
then the Grid installation path is changed to root