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.

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.

A package is a collection of related PL/SQL objects. The package contains a body and a specification. The package specification has the declaration which is public and can be used in the PL/SQL sub programs inside the package.

The package body holds the implementation of all the PL/SQL objects declared in the specification.

Example of a PL/SQL Package.

CREATE OR REPLACE PACKAGE emp_data AS 

PROCEDURE add_employee (

      ename VARCHAR2,

      job VARCHAR2,

      mgr NUMBER,

      sal NUMBER,

      deptno NUMBER);

END emp_actions;

CREATE OR REPLACE PACKAGE BODY emp_data AS 

PROCEDURE add_employee (

       ename VARCHAR2,

       job VARCHAR2,

       mgr NUMBER,

       sal NUMBER,

       deptno NUMBER) IS

BEGIN

         INSERT INTO emp VALUES (empno_seq.NEXTVAL, ename, job, mgr, SYSDATE, comm, deptno);

END add_employee;

END emp_data;

Advantages of  PL/SQL packages :

Packages are easier for application designing, encapsulating data, additional functionality and better performance. An application has various modules which can be placed in packages and handled easier.

Named PL/SQL blocks of code which can be invoked using parameters are called PL/SQL sub programs.

Advantages of PL/SQL subprograms are

- The application makes a single call to the database to run a block of statements which improves performance against running SQL multiple times. This will reduce the number of calls between the database and the application.

- PL/SQL is secure since the code resides inside the database thus hiding internal database details from the application. The application will only make a call to the PL/SQL sub program

- PL/SQL and SQL go hand in hand so there would be no need of any translation required between PL/SQL and SQL.

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;

1)Divide by zero        – This is raised when any number is attempted to divide by zero.

2)TOO MANY ROWS   - A SELECT INTO statement returns more than one row.

3)CASE_NOT_FOUND - No choice in the WHEN clause of a case statement is selected.

4)LOGIN_DENIED      - An attempt to login with an invalid username or password.

5)PROGRAM_ERROR  - An internal PL/SQL problem.

Cursors help you manipulate the information retrieved by select statements. This can be done by assigning a name to the cursor.

Example:

CURSOR emp_cur 

IS 

SELECT emp_number from employee_tbl where employee_name = name_in;

Types of Cursors:

Implicit cursors- These cursors are not declared by the programmer. They are issued when the SQL statement is executed. The open, close and fetching is done by itself.

Example:

UPDATE employee SET salary = salary * 2.1;

Here, an implicit cursor is issued to identify the set of rows in the table which would be affected by the update.

Explicit cursors- These cursors are defined by programmer. They are used in queries that return multiple rows.

Example:

CURSOR emp_cur 

IS 

SELECT emp_number from employee_tbl where employee_name = name_in;

A record data type represents a data type for that row in a database table. It lets u define your own records and not your own fields.