Dynamic references to trigger vars :new and :old

Question

Is there any way to dynamicly refer to the :old and :new bind
variables available in triggers?
I'm looking for a method to make a standard processing system
that processes based on column-names. I want to avoid calling my
handling system staticly for each field in the table.
Ie. I want to refer to :old.data_column in a way where I can
loop through all the fields of the table.
Hints, tips and ideas would be greatly apprechiated.

Answer 1

You can't have dynamic PL/SQL containing ":NEW" or ":OLD" in
database triggers.
But in some cases, if your table has primary key (PK)
or unique key (UK), you can use AFTER STATEMENT triggers
in following way
(this is like well-known solution for mutating error problem):
1. create database package with PL/SQL table
(for storing PK values)
2. clear PL/SQL table in BEFORE STATEMENT trigger
3. populate PL/SQL table with PK value
in BEFORE (or AFTER) ROW trigger
4. read rows in AFTER STATEMENT trigger with dynamic PL/SQL,
using PKs from PL/SQL table and using dynamically selected
column names with statement
SELECT column_name
FROM user_tab_columns
WHERE UPPER (table_name) = UPPER (p_table);"
(you can read NEW values of columns and,
with AUTONOMOUS_TRANSACTION, OLD values too)
For example (scott.dept table):
CREATE OR REPLACE TRIGGER bus_dept
BEFORE UPDATE ON dept
BEGIN
plsql_table.clear;
END;
CREATE OR REPLACE TRIGGER bur_dept
BEFORE UPDATE ON dept
FOR EACH ROW
BEGIN
plsql_table.populate_with_id (:NEW.deptno);
END;
CREATE OR REPLACE TRIGGER aus_dept
AFTER UPDATE ON dept
DECLARE
v_current_id dept.deptno%TYPE;
BEGIN
dynamic_new_old.set_table_name ('dept');
dynamic_new_old.set_pk_name ('deptno');
dynamic_new_old.create_column_names;
WHILE plsql_table.id_exists LOOP
v_current_id := plsql_table.current_id;
DBMS_OUTPUT.PUT_LINE ('OLD VALUES:');
dynamic_new_old.display_old_values (v_current_id);
DBMS_OUTPUT.PUT_LINE ('NEW VALUES:');
dynamic_new_old.display_new_values (v_current_id);
DBMS_OUTPUT.PUT_LINE ('*****');
END LOOP;
END;
CREATE OR REPLACE PACKAGE plsql_table IS
PROCEDURE clear;
PROCEDURE populate_with_id (p_id dept.deptno%TYPE);
FUNCTION id_exists RETURN BOOLEAN;
FUNCTION current_id RETURN dept.deptno%TYPE;
END;
CREATE OR REPLACE PACKAGE BODY plsql_table IS
TYPE type_plsql_table IS TABLE OF dept.deptno%TYPE INDEX BY
BINARY_INTEGER;
m_plsql_table type_plsql_table;
-- prefiks m_ is for module level variable (defined in package
body)
-- prefiks g_ is for global variable (defined in package
specification)
m_rec_number BINARY_INTEGER;
PROCEDURE clear IS
BEGIN
m_rec_number := 0;
END;
PROCEDURE populate_with_id (p_id dept.deptno%TYPE) IS
BEGIN
m_rec_number := m_rec_number + 1;
m_plsql_table (m_rec_number) := p_id;
END;
FUNCTION id_exists RETURN BOOLEAN IS
BEGIN
RETURN (m_rec_number > 0);
END;
FUNCTION current_id RETURN dept.deptno%TYPE IS
v_id dept.deptno%TYPE;
BEGIN
v_id := m_plsql_table (m_rec_number);
m_rec_number := m_rec_number - 1;
RETURN v_id;
END;
END;
CREATE OR REPLACE PACKAGE dynamic_new_old IS
PROCEDURE set_table_name (p_table VARCHAR2);
PROCEDURE set_pk_name (p_pk VARCHAR2);
PROCEDURE create_column_names;
PROCEDURE display_old_values (p_id dept.deptno%TYPE);
PROCEDURE display_new_values (p_id dept.deptno%TYPE);
END;
CREATE OR REPLACE PACKAGE BODY dynamic_new_old IS
m_table VARCHAR2 (30);
m_pk VARCHAR2 (30);
m_columns VARCHAR2 (1000);
PROCEDURE set_table_name (p_table VARCHAR2) IS
BEGIN
m_table := p_table;
END;
PROCEDURE set_pk_name (p_pk VARCHAR2) IS
BEGIN
m_pk := p_pk;
END;
PROCEDURE create_column_names IS
v_first_column BOOLEAN;
BEGIN
v_first_column := TRUE;
FOR rec IN
(SELECT column_name
FROM user_tab_columns
WHERE UPPER (table_name) = UPPER (m_table))
LOOP
IF v_first_column THEN
v_first_column := FALSE;
m_columns := 'v_record.' || rec.column_name;
ELSE
m_columns := m_columns ||
'||' || '''--''' || '|| v_record.' || rec.column_name;
END IF;
END LOOP;
END;
PROCEDURE display_values (p_id dept.deptno%TYPE) IS
v_cursor INTEGER;
v_rows_processed INTEGER;
v_statement VARCHAR2 (32000);
BEGIN
v_statement :=
' DECLARE ' ||
' v_record ' || m_table || '%ROWTYPE;' ||
' BEGIN' ||
' SELECT * INTO v_record' ||
' FROM ' || m_table ||
' WHERE ' || m_pk || ' = ' || p_id || ';' ||
' DBMS_OUTPUT.PUT_LINE (' || m_columns || ');' ||
' END;';
v_cursor := DBMS_SQL.OPEN_CURSOR;
DBMS_SQL.PARSE (v_cursor, v_statement, DBMS_SQL.V7);
v_rows_processed := DBMS_SQL.EXECUTE (v_cursor);
DBMS_SQL.CLOSE_CURSOR (v_cursor);
EXCEPTION
WHEN OTHERS THEN
DBMS_OUTPUT.PUT_LINE (sqlerrm);
IF DBMS_SQL.IS_OPEN (v_cursor) THEN
DBMS_SQL.CLOSE_CURSOR (v_cursor);
END IF;
END;
PROCEDURE display_old_values (p_id dept.deptno%TYPE) IS
PRAGMA AUTONOMOUS_TRANSACTION;
BEGIN
display_values (p_id);
END;
PROCEDURE display_new_values (p_id dept.deptno%TYPE) IS
BEGIN
display_values (p_id);
END;
END;
Note that this code is not generic, because uses
"dept.deptno%TYPE". If all your PKs has the same declaration
(for example NUMBER), then you can write generic solution.
If you need only OLD values, you can write a simpler solution
(without statement triggers and "plsql_table" package),
using "dynamic_new_old" package and AUTONOMOUS_TRANSACTION
in BEFORE (or AFTER) ROW trigger.
Regards
Zlatko Sirotic