Audit history of multiple tables in the database

Question

I have seen a very effective implementation of this which goes as follows:

TABLE audit_entry (
    audit_entry_id          INTEGER         PRIMARY KEY,
    audit_entry_type        VARCHAR2(10)    NOT NULL,
    -- ^^ stores 'INSERT' / 'UPDATE' -- / 'DELETE'

    table_name              VARCHAR2(30)    NOT NULL,
    -- ^^ stores the name of the table that is changed

    column_name             VARCHAR2(30)    NOT NULL,
    -- ^^ stores the name of the column that is changed

    primary_key_id          INTEGER         NOT NULL,
    -- ^^ Primary key ID to identify the row that is changed

    -- Below are the actual values that are changed.
    -- If the changed column is a foreign key ID then
    -- below columns tell you which is new and which is old
    old_id                  INTEGER,
    new_id                  INTEGER,

    -- If the changed column is of any other numeric type,
    -- store the old and new values here.
    -- Modify the precision and scale of NUMBER as per your 
    -- choice.
    old_number              NUMBER(18,2),
    new_number              NUMBER(18,2),

    -- If the changed column is of date type, with or without
    -- time information, store it here.
    old_ts                  TIMESTAMP,
    new_ts                  TIMESTAMP,

    -- If the changed column is of VARCHAR2 type,
    -- store it here.
    old_varchar             VARCHAR2(2000),
    new_varchar             VARCHAR2(2000),
    ...
    ... -- Any other columns to store data of other types,
    ... -- e.g., blob, xmldata, etc.
    ...
)

And we create a simple sequence to give us new incremental integer value for audit_entry_id:

CREATE SEQUENCE audit_entry_id_seq;

The beauty of a table like audit_entry is that you can store information about all types of DMLs- INSERT, UPDATE and DELETE in the same place.

For e.g., for insert, keep the old_* columns null and populate the new_* with your values.

For updates, populate both old_* and new_* columns whenever they are changed.

For delete, just populate the old_* columns and keep the new_* null.

And of course, enter the appropriate value for audit_entry_type. ;0)

Then, for example, you have a table like follows:

TABLE emp (
    empno           INTEGER,
    ename           VARCHAR2(100) NOT NULL,
    date_of_birth   DATE,
    salary          NUMBER(18,2) NOT NULL,
    deptno          INTEGER -- FOREIGN KEY to, say, department
    ...
    ... -- Any other columns that you may fancy.
    ...
)

Just create a trigger on this table as follows:

CREATE OR REPLACE TRIGGER emp_rbiud
-- rbiud means Row level, Before Insert, Update, Delete
BEFORE INSERT OR UPDATE OR DELETE
ON emp
REFERENCING NEW AS NEW OLD AS OLD
DECLARE
    -- any variable declarations that deem fit.
BEGIN
    WHEN INSERTING THEN
        -- Of course, you will insert empno.
        -- Let's populate other columns.

        -- As emp.ename is a not null column, 
        -- let's insert the audit entry value directly.
        INSERT INTO audit_entry(audit_entry_id,
                                audit_entry_type,
                                table_name,
                                column_name,
                                primary_key,
                                new_varchar)
        VALUES(audit_entry_id_seq.nextval,
               'INSERT',
               'EMP',
               'ENAME',
               :new.empno,
               :new.ename);

        -- Now, as date_of_birth may contain null, we do:
        IF :new.date_of_birth IS NOT NULL THEN
            INSERT INTO audit_entry(audit_entry_id,
                                    audit_entry_type,
                                    table_name,
                                    column_name,
                                    primary_key,
                                    new_ts)
            VALUES(audit_entry_id_seq.nextval,
                   'INSERT',
                   'EMP',
                   'DATE_OF_BIRTH',
                   :new.empno,
                   :new.date_of_birth);
        END IF;

        -- Similarly, code DML statements for auditing other values
        -- as per your requirements.

    WHEN UPDATING THEN
        -- This is a tricky one.
        -- You must check which columns have been updated before you
        -- hurry into auditing their information.

        IF :old.ename != :new.ename THEN
            INSERT INTO audit_entry(audit_entry_id,
                                    audit_entry_type,
                                    table_name,
                                    column_name,
                                    primary_key,
                                    old_varchar,
                                    new_varchar)
            VALUES(audit_entry_id_seq.nextval,
                   'INSERT',
                   'EMP',
                   'ENAME',
                   :new.empno,
                   :old.ename,
                   :new.ename);
        END IF;

        -- Code further DML statements in similar fashion for other
        -- columns as per your requirement.

    WHEN DELETING THEN
        -- By now you must have got the idea about how to go about this.
        -- ;0)
END;
/

Just one word of caution: be selective with what tables and columns you choose to audit, because anyways, you this table will have a huge number of rows. SELECT statements on this table will be slower than you may expect.

I would really love to see any other sort of implementation here, as it would be a good learning experience. Hope your question gets more answers, as this is the best implementation of an audit table that I have seen and I'm still looking for ways to make it better.