Declaring PL/SQL variables and constants in Oracle

Example of declaring Variableiables:

Variable1 varchar2(100);
Join_date Date;
Variable2 number default 5;
Variable3 number not null := 2;

Not Null means a value may change but it can never be assigned Null.

Variable4 varchar2(20) := Null;
Variable5 varchar2(20) default Null;

Example of declaring Constants:

Variable_constant constant number := 100;

Constants cannot be changed.

You must initialize constants at the time of declaration.