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In prior releases, it was actually possible to do a CASCADE UPDATE, but doing so involved a tremendous amount of work and had certain limitations. With deferrable constraints, it becomes almost trivial. The code could look like this: ops$tkyte%ORA11GR2> create table parent 2 ( pk int primary key ) 3 / Table created. ops$tkyte%ORA11GR2> create table child 2 ( fk constraint child_fk_parent 3 references parent(pk) 4 deferrable 5 initially immediate 6 ) 7 / Table created. ops$tkyte%ORA11GR2> insert into parent values ( 1 ); 1 row created. ops$tkyte%ORA11GR2> insert into child values ( 1 ); 1 row created. We have a parent table, PARENT, and a child table, CHILD. Table CHILD references table PARENT, and the constraint used to enforce that rule is called CHILD_FK_PARENT (child foreign key to parent). This constraint was created as DEFERRABLE, but it is set to INITIALLY IMMEDIATE. This means we can defer that constraint until COMMIT or to some other time. By default, however, it will be validated at the statement level. This is the most common use of the deferrable constraints. Most existing applications won t check for constraint violations on a COMMIT statement, and it is best not to surprise them with that. As defined, table CHILD behaves in the fashion tables always have, but it gives us the ability to explicitly change its behavior. Now let s try some DML on the tables and see what happens:

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It surprises many people to discover that a connection is not synonymous with a session In most people s eyes they are the same, but the reality is they do not have to be A connection may have zero, one, or more sessions established on it Each session is separate and independent, even though they all share the same physical connection to the database A commit in one session does not affect any other session on that connection In fact, each session using that connection could use different user identities! In Oracle, a connection is simply a physical circuit between your client process and the database instance a network connection, most commonly The connection may be to a dedicated server process or to a dispatcher As previously stated, a connection may have zero or more sessions, meaning that a connection may exist with no corresponding sessions.

Sharing of data: Any number of applications with the appropriate access rights can connect to your database and read the data stored within all without the need to worry about containing the logic to extract this data As you will see shortly, applications use various query languages (most notably SQL) to communicate with databases..

Additionally, a session may or may not have a connection Using advanced Oracle Net features such as connection pooling, a physical connection may be dropped by a client, leaving the session intact (but idle) When the client wants to perform some operation in that session, it would reestablish the physical connection Let s define these terms in more detail: Connection: A connection is a physical path from a client to an Oracle instance A connection is established either over a network or over an IPC mechanism A connection is typically between a client process and either a dedicated server or a dispatcher However, using Oracle s Connection Manager (CMAN), a connection may be between a client and CMAN, and CMAN and the database Coverage of CMAN is beyond the scope of this book, but Oracle Net Services Administrator s Guide (freely available from http://otnoracle.

A logical organization of data: You can write new applications that work with the same data without having to worry about how the data is physically represented and stored. On the basic level, this logical structure is given by a set of entities (data tables) and their relationships. Avoiding data redundancy: Having all requirements from each consuming application up front helps to identify a logical organization for your data that minimizes possible redundancy. For instance, you can use foreign keys instead of duplicating pieces of data. Data normalization is the process of systematically eliminating data redundancy, a large but essential topic that we don t consider in this book. Transactions: Reading from and writing to databases occurs atomically, and as a result two concurrent transactions can never leave data in an inconsistent, inaccurate state. Isolation levels refer to the specific measures taken to ensure transaction isolation by locking various parts of the database (fields, records, tables). Higher isolation levels increase locking overhead and can lead to a loss of parallelism by rendering concurrent transactions sequential; on the other hand, no isolation can lead to inconsistent data. Maintaining data integrity: In other words, databases make sure that the data stored within is accurate. Having no redundancy is one way to maintain data integrity (if a piece of data is changed, it is changed in the only place it occurs; thus, it remains accurate); on the other hand, data security and transaction isolation are needed to ensure that the data stored is modified in a controlled manner.