Re: how to query the structure of a database

From: David Cressey <>
Date: Wed, 30 Jan 2002 12:33:48 GMT
Message-ID: <MKR58.268$>

> If I remember well Dr. Codd wrote that a DBMS has to store not only the
> but the metadata too,
> and the metadata have to be retrieved with the same methods used for te

This is true. As the 12 rules were presented to me, one of them says that the metadata has to be stored
in relations (tables) just as the data is. It further states that the query mechanism, e.g. SQL SELECT,
that is used for composing data queries can also be used for composing metadata queries.

All of the commercial relational DBMS systems I know of conform to this requirement, although in some cases the metadata schema is less comprehensive than it might be.

The problem is that they don't all use the same schema. There is more than one way to model tables, columns, domains, and indexes. There is more than one way to describe them in a metadata schema. And this is where there is no standard at present. Other responses have already outlined how later versions of the SQL standard address this issue. But those remedies have yet to filter into the commercial marketplace.

At least, that's what I think I see.

    David Cressey
Received on Wed Jan 30 2002 - 13:33:48 CET

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