In PL/SQL, variables must be included in the declaration block before they can be used.
There are a number of ways to declare a variable.
The most common way is by using a direct declaration, as shown here:
declare
variable_name [constant] DATATYPE [DEFAULT value |DEFAULT NULL];
begin
...
The keyword constant means that the variable's value can't be changed in the body of the program.
If you declare a variable as a constant, you must assign a default value to it by using the optional DEFAULT value clause.
The following shows an example of correct declarations of variables:
SQL> declare
2 v_sal NUMBER;
3 v_name VARCHAR2(10) DEFAULT 'KING';
4 v_start_dt DATE := SYSDATE; -- same as DEFAULT SYSDATE
5 begin
6 NULL;
7 end;
8 /
PL/SQL procedure successfully completed.
SQL>