Block Structure approach

Block Structure approach:

• The basic units that make up a PL/SQL program are logical blocks.
• The block in PL/SQL can be nested with one another.
• A block groups related declarations and statements into one single unit.

The basic parts of a PL/SQL block are:

1. Declarative part (optional).
2. Executable part (Mandatory).
3. Exception handling (Optional).

1.Declarative part:It is used to define user defined types, variables which can be used, in the executable part for further manipulations. 

2.Executable part:
All  procedural statements are included between the BEGIN and END statement.
It must have one executable statement.

3.Exception Handling part:
Error that occur during execution are dealt in the exception handling part.

Notes:

• A PL/SQL program is a logical block, which contain any number of nested sub blocks.
• Block can be nested in the executable and exception handling parts of a PL/SQL block, or a sub program.
• A PL/SQL is marked with either a "DECLARE" or "BEGIN" keyword and ends with the keyword "END".
• Only BEGIN and END keywords are mandatory.
• A semicolon(;) has to be placed after the "END" keyword.

Block syntax:
SQL> DECLARE
    [Variable declarations;
    Cursor declaration;
    User_defined exception;]
          BEGIN
          <SQL statements;>
          PL/SQL statement
          EXCEPTION
          Action to perform when errors occurs.
          END;

• Only executable section is required.
• The declarative and exception handling section are optional.
• We can define local sub programs in the declarative part of any block.
• However, we can call local subprograms only from the block in which they are define.