Possible Causes of “ORA-06502: PL/SQL: numeric or value error”

Sometimes people ask me that what the reason of ORA-06502 is. Mostly, this is because of carelessness of developersJ. Official description of ORA-06502 is

[oracle@dhcppc5 ~]$ oerr ora 6502

06502, 00000, "PL/SQL: numeric or value error%s"

// *Cause: An arithmetic, numeric, string, conversion, or constraint error

//         occurred. For example, this error occurs if an attempt is made to

//         assign the value NULL to a variable declared NOT NULL, or if an

//         attempt is made to assign an integer larger than 99 to a variable

//         declared NUMBER(2).  

// *Action: Change the data, how it is manipulated, or how it is declared so

//          that values do not violate constraints.

[oracle@dhcppc5 ~]$

This exception also exists in the STANDART package of Oracle database:

  VALUE_ERROR exception;

    pragma EXCEPTION_INIT(VALUE_ERROR, '-6502');

I show possible reasons of this error below:

DECLARE

  i  NUMBER;

  i2 NUMBER NOT NULL := 0;-- !is not a best practice...

  i3 NUMBER(1);

  i4 POSITIVE;

  s  VARCHAR2(3) DEFAULT 'aaa';-- !is not a best practice...

  s2 VARCHAR2(1);

BEGIN

  -- assign to a number data type variable a non-number value

  i := s; -- raises ORA-06502: PL/SQL: numeric or value error: character to number conversion error

  -- assign more values (in terms of length) to a variable with less size

  s2 := s; -- raises ORA-06502: PL/SQL: numeric or value error: character string buffer too small

  -- assign a not-null-constrained value to null value

  i2 := i; -- raises ORA-06502: PL/SQL: numeric or value error

  -- assign bigger numeric value to a smaller size numeric variable

  i3 := 10; -- raises  ORA-06502: PL/SQL: numeric or value error: number precision too large

  -- assign negative value to positive data type

  i4 := -1; -- raises  ORA-06502: PL/SQL: numeric or value error

END;