PL/SQL Block Structure

PL/SQL is block structured language divided into three logical blocks.
BEGIN block and END; keyword are compulsory, and other two block DECLARE and EXCEPTION are optional block. END; is not a block only keyword to end of PL/SQL program.
PL/SQL block structure follows divide-and-conquer approach to solve the problem stepwise.

Figure - PL/SQL block Structure

DECLARE

Variables and constants are declared, initialized within this section.
Variables and Constants : In this block, declare and initialize variables (and constants). You must have to declare variables and constants in declarative block before referencing them in procedural statement.
Declare Variables and Assigning values : You can define variable name, data type of a variable and its size. Date type can be: CHAR, VARCHAR2, DATE, NUMBER, INT or any other.

DECLARE    -- DECLARE block, declare and initialize values
 designation VARCHAR2(30);
 eno number(5) := 5;
 id BOOLEAN;
    inter INTERVAL YEAR(2) TO MONTH;
BEGIN     -- BEGIN block, also assign values  
 designation := UPPER('Web Developer');
 id := TRUE;
 inter := INTERVAL '45' YEAR; 
END;
/

Declare Constants and Assigning values : Constants are declared same as variable but you have to add the CONSTANT keyword before defining data type. Once you define a constant value you can't change the value.

designation CONSTANT VARCHAR2(30) := 'Web Developer';

BEGIN

BEGIN block is procedural statement block which will implement the actual programming logic. This section contains conditional statements (if...else), looping statements (for, while) and Branching Statements (goto) etc.

EXCEPTION

PL/SQL easily detects user defined or predefined error condition. PL/SQL is famous for handling errors in smart way by giving suitable user friendly messages. Errors can be rise due to wrong syntax, bad logical or not passing a validation rules.
You can also define exception in your declarative block and later you can execute it by RAISE statement.

DECLARE
 check_exist EXCEPTION; -- declare exception type
 ...
BEGIN
 ....
 RAISE check_exist;  -- Raise exception
 ....
EXCEPTION
   WHEN check_exist THEN  -- execute raise exception 
   ..... 
END;
/

Note :

1. BEGIN block and END; keyword are compulsory of any PL/SQL program.
2. Where as DECLARE and EXCEPTION block are optional.