Oracle get values from :new, :old dynamically by string key - oracle

Question

How to get a value from special :new or :old by "string key"?
e.g. in PHP:
$key = 'bar';
$foo[$key]; //get foo value
How to in Oracle?
:new.bar --get :new 'bar' value
and
key = 'bar';
:new[key] --How to?
Is it possible?
Thx!

Answer 1

It is not possible.
A trigger that fires at row level can access the data in the row that
it is processing by using correlation names. The default correlation
names are OLD, NEW, and PARENT.
...
OLD, NEW, and PARENT are also
called pseudorecords, because they have record structure, but are
allowed in fewer contexts than records are. The structure of a
pseudorecord is table_name%ROWTYPE, where table_name is the name of
the table on which the trigger is created (for OLD and NEW) or the
name of the parent table (for PARENT).
http://docs.oracle.com/cd/E11882_01/appdev.112/e25519/triggers.htm#autoId4
So, these correlation names are basically records. Record is not a key-value storage so you cannot reference its by string key.
Here's what you can do with them:
http://docs.oracle.com/cd/E11882_01/appdev.112/e10472/composites.htm#CIHFCFCJ

Answer 2

According to this
The first approach is syntactically good, it is to to be used like this:
create trigger trg_before_insert before insert on trigger_tbl
for each row
begin
insert into trigger_log (txt) values ('[I] :old.a=' || :old.a || ', :new.a='||:new.a);
end;
/
But if you want to access the field dynamically, one ugly thing that I can think of, that seems to be working (which is not dynamic in the end at all...): using a CASE WHEN... statement for each column you want to be able to use dynamically...
Something along these lines (updating the :new record):
key='bar';
value = 'newValue';
CASE key
WHEN 'bar' THEN :new.bar = value;
WHEN 'foo' THEN :new.foo = value;
WHEN 'baz' THEN :new.baz = value;
END;
To read a value from "dynamic column":
key='bar';
value = CASE key
WHEN 'bar' THEN :new.bar;
WHEN 'foo' THEN :new.foo;
WHEN 'baz' THEN :new.baz;
END;
Then use the value variable as required...
Beware however, as #beherenow noted:
what's the datatype of value variable in your reading example?
and how can you be sure you're not going to encounter a type mismatch?
These are questions that require decisions from the implementing side. For example, with a squint, this thing could be used to dynamically use values from columns that share the same type.
I have to emphasize though, that I don't see a situation where such a bizarre contraption I proposed is to be used, nor do I support using this. The reason I kept it here, after #beherenow's complete and definitive answer is so that everyone finding this page can see - though there might be a way, it shouldn't be used...
To me, this thing seems:
ugly
brittle
badly scaling
appalling
difficult to maintain
...aaand horribly ugly...
I definitely recommend rethinking the use case you need this for. I myself would angrily shout with someone writing this kind of code, unless this is absolutely the only way, and the whole universe collapses, if this is not done this way... (This is very much unlikely though)
Sorry if I misunderstood your question, it was not totally clear to me