Oracle Questions

Oracle constraints are used to maintain consistent of data and ensure the data is properly maintained. A constraint is more or less a restriction we try to apply on a table.
Types of constraints:
- Check constraints
- NOT NULL constraint
- PRIMARY KEY constraint
- REFERENCES constraint
- UNIQUE constraint

A sequence is a database object that is used to generate sequential number.

CREATE SEQUENCE seqname [increment] [minimum value][maximum value][start][cache][cycle]               Nextval and currval lets us get the next value and current value from the sequence.

PL/SQL exceptions are raised using the RAISE command. This command is used when exceptions are defined by programmer and not implicit exceptions.

Example:

Declare and raising an exception:

DECLARE

short_of_attendance EXCEPTION;

min_attendance NUMBER(4);

BEGIN

...

IF min_attendance < 10 THEN

RAISE short_of_attendance;

END IF;

EXCEPTION

WHEN short_of_attendance THEN

-- handle the error

END;

Dropped tables can be recovered using DROP TABLE flashback. It works the way recycle bin works.
Example:
FLASHBACK TABLE EMPLOYEE TO BEFORE DROP;
The most recently dropped table with that original name is retrieved from the recycle bin, with its original name.

A database trigger is procedural code that is automatically executed in response to certain events on a particular table or view in a database. The trigger is mostly used for maintaining the integrity of the information on the database.

Syntax:

CREATE OR REPLACE TRIGGER [Trigger Name] [Before / After / Instead Of]

ON [schema].[table]

<PL/SQL subprogram>

Types of PL/SQL triggers : 

> Row trigger          - The trigger fires for each ROW affected.

> Statement trigger - The trigger is fired once when the condition is matched

> Before and After trigger - The BEFORE trigger run the trigger action before the insert, update or delete statement. The AFTER trigger runs the trigger action after the insert, update or delete statement is executes.

- Mask the real name and owner of an object.
- Provide public access to an object
- Provide location transparency for tables, views or program units of a remote database.
- Simplify the SQL statements for database users.

It is virtual table which is defined as a stored procedure based on one or more tables.

- These are the Logical Storage Units into which a database is divided.
- It is used to group together the related logical structures.