Multitenancy is a new concept in Oracle 12c. The basis of it is that, you can separate the application data from the system data. Up to and including 11g, all the data in the database was residing in the same dimension as the system data or any other data for that matter. Data related to different application, were in the same tablespaces or even managed by the same user. Well, from 12c, all this can go under the table, now you can have many databases PDBs (pluggable databases), inside one big database, the root container. There are even more possibilities like application containers and so on, but we will see that later.

With the multi-tenancy, we have a:

• root container - Where the system data resides. Also you can choose if you want the UNDO tablespace in the root to be used for all PDBs, or each PDB to have its own undo, SYSTEM and SYSAUX tablespace, making it completely independent from the root container.
• Application Container - Container which consist of 1 or many PDBs which host a specific application. We are able to assign versions and name to such container, furthermore we can initiate installation and upgrade modes.

Firstly, we need to create our container. From 12c, we can create a database to be either as it always has been, a normal database, OR we can create it to allow other databases as well. So it will be a container, let's see how we can do both:

Actually, the creation of a normal database isn't that hard, you can just type:

SQL> Create database;

Simply that, will create us a normal database, well after that you will have to run the catproc and the catalog scripts, but you get the gist.

To create a container, database we NEED to allow it to have PLUGGABLE databases, as follows:

SQL> CREATE DATABASE CDB1 ENABLE PLUGGABLE DATABASE SEED FILE_NAME_CONVERT = ('/loc1/data','/loc2/data');

That will create us a container database called CDB1 and a SEED, after that you will have to run script:

@$ORACLE_HOME/rdbms/admin/catcdb.sql

But other than that, all will be ok. So we have a container, big deal, what then. Well, now we can create PLUGGABLE databases as follows:

As I said before, pluggable databases, are totally separate databases from one another. Imagine it having many databases (PDBs) within one big database (CDB). These PDBs, can have separate user totally individual from each other. So let's see how it is done:

CREATE PLUGGABLE DATABASE pdb2 ADMIN USER pdb_adm IDENTIFIED BY Password1
  FILE_NAME_CONVERT=('/u01/app/oracle/oradata/cdb1/pdbseed/','/u01/app/oracle/oradata/cdb1/pdb2/');

You can use either:

• FILE_NAME_CONVERT, or
• CREATE_FILE_DEST, or
• PDB_FILE_NAME_CONVERT

Either in the session level or on the CREATE statement, up to you, the result will be the same.

Once created, you can list all PDBs in a certain CDB as follows:

COLUMN pdb_name FORMAT A20

SELECT pdb_name, status
FROM   dba_pdbs
ORDER BY pdb_name;

PDB_NAME	     STATUS
-------------------- -------------
PDB$SEED	     NORMAL
PDB1		     NORMAL
PDB2		     NEW
PDB3		     NEW

COLUMN name FORMAT A20

SELECT name, open_mode
FROM   v$pdbs
ORDER BY name;

NAME			       OPEN_MODE
------------------------------ ----------
PDB$SEED		       READ ONLY
PDB1			       MOUNTED
PDB2			       MOUNTED
PDB3			       MOUNTED

Remember, 1 PDB can be in only 1 ROOT$CDB, but 1 CDB can have many PDBs. Furthermore, from 12.2, we have also applicational containers, which are containers within the CDB, but we will speak about them in a second.