Re: How to create a DBMS from scratch?

The world rejoiced as emmettwa_at_onebox.com (Emmett) wrote:
> Anthony.Youngman_at_ECA-International.com (Anthony Youngman) wrote in message news:<9a993dee.0211190404.1d0e356c_at_posting.google.com>...
> I figure the problems being referred to in this post have to do with
> making XPath queries available on data - not in storing XML.

That's probably about the size of it.

There's precisely nothing preventing people from storing XML data in an RDBMS VARCHAR field.

The challenge comes in treating the set of data as a hierarchy and invoking hierarchy-related queries in an intelligent way.

There are two fundamental issues vis-a-vis SQL:

1. SQL queries return sets that are not inherently ordered, which means that there's not a totally natural mapping of ordered hierarchical data onto the DBMS;
2. A set is not a tree, which is kind of the converse of a). SQL queries return sets, so that you can't get a tree out of a query.

You can't put in a tree /as a tree/, and can't get it out /as a tree/... That makes using XML a bit troublesome.

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