Check digit function

It is a good idea to have a check digit in ID columns, especially if values are entered manually. Here is a check function to use:

CREATE OR REPLACE  FUNCTION add_check_digit( in_val IN NUMBER ) RETURN NUMBER AS
   c_in_val VARCHAR2(100);
   c_sum  NUMBER := 0; 
   i NUMBER;
   BEGIN
      c_in_val := TO_CHAR( in_val );                
      i := LENGTH( c_in_val );
      WHILE i > 0 LOOP
         c_sum := c_sum + TO_NUMBER( SUBSTR( c_in_val, i, 1 ) );
         i := i - 1;
      END LOOP; 
   RETURN TO_NUMBER( c_in_val || MOD( c_sum, 10 ) );
END add_check_digit;
/
SHOW ERRORS

Then create the table:

CREATE TABLE a_table ( id_col number(10) );

Here is a trigger to enforce the rule, since Oracle doesn't allow self-defined functions in check-constraints:

CREATE OR REPLACE TRIGGER ar_iu_a_table 
     AFTER INSERT 
        OR UPDATE OF id_col
     ON a_table 
     FOR EACH ROW 
BEGIN
  IF :new.id_col != add_check_digit( TRUNC( :new.id_col / 10 ) ) THEN
    RAISE_APPLICATION_ERROR( -20000, 'id_col failed check-digit-test !' );
  END IF;
END;
/
SHOW ERRORS

Finally, here is how to use the function for generating new datasets:

INSERT INTO a_table ( id_col) VALUES ( add_check_digit( your_id_sequence.nextval() ) )

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This was first published in May 2002

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