Syntax:

      Identifier[CONSTANT]Data type[NOT NULL][:= DEFAULT Expression];
 
  => Identifier- Specifies the name of the variables.
  => Constant  - Constraints the variables such that its value cannot change. Constants must be Initialized.
  => Data type  - It is a Scalar,composite,reference or LOB Datatype.
  => NOT NULL  - Constraints a variable such that it must contain a value. Should be initialized.
  => Expression- It is any PL/SQL Expression that can be a literal, another variable or an expression involving operations and functions.