Drupal CCK delta value and other mysteries

For my aim to create an import-script (see other blog entry), I had to go deeper into Drupal DB values.

I don't like to do things again and again and I hope, that this note will be useful for somebody else perhaps, too.

My understanding of the common drupal cck-columns is:

"delta": Value for sorting the fields
==> 0 = first value
==> 1 = second value
... and so on.

Another aspect that is hard to understand, is the ..._data - column.
It seems to break the rules for database normalization and sets several values as a list in one column.

For example:
a:2:{s:3:"alt";s:0:"";s:5:"title";s:0:"";}
(CCK Image field)

As I understand this, it seems to encapsulate information about which data to use for the (html-)attributes.

An empty entry would look like this:
'a:0:{}'

From my point of view this code really smells bad. It is hard to understand, not db-normalized, ...

Technically it is just a serialized array, for example:

PHP:
print_r(serialize(array('a'=>'value a',
'b'=>'value b')));

RESULT:
a:2:{s:1:"a";s:5:"value a";s:1:"b";s:4:"value b";}

Anyway, we have to struggle with it and my oppinion doesn't matter in this case. For my script I will just leave this empty ('a:0:{}') and see, if it will cause problems. At the moment I think, there's no other possibility, because I can't find a documentation about this. So if you know better, please let me know!