Yes!! it is very cool feature in SQL. I used them
very often and it saves hell of time!! The reason
I can do 'Alter domain' to change char(10) to char(30)
and it is propogated in all the tables automatically
whereever domain is used!! Also need to have 'ALTER DOMAIN' very
---Tom I Helbekkmo <tih(at)Hamartun(dot)Priv(dot)NO> wrote:
> On Sun, Mar 01, 1998 at 03:01:12PM -0500, The Hermit Hacker wrote:
> > > The datatype employed is defined by domain which also
> > > restricts the values to "YES" or "NO" or "RETIRED" or "DISABLED"
> > > NULL.
> > Oh, cool...so, essentially, you are creating an enumerated(?) type
> > to be used in a table?
> Cool indeed! Actually, a domain definition can be useful for more
> than just that: if you define a domain, and then use that domain as a
> data type for various columns in various tables, you can change your
> schema all at once by changing the definition of the domain. Also, a
> domain can carry extra meaning. Look at this schema (using a somewhat
> arcane syntax) for keeping track of suppliers, parts and shipments of
> quantities of parts from suppliers:
> DOMAIN S# CHARACTER (5) PRIMARY
> DOMAIN SNAME CHARACTER (40)
> DOMAIN P# CHARACTER (5) PRIMARY
> DOMAIN PNAME CHARACTER (20)
> RELATION S (S#, SNAME)
> PRIMARY KEY (S#)
> RELATION P (P#, PNAME)
> PRIMARY KEY (P#)
> RELATION SP (S#, P#, QTY NUMERIC (4))
> PRIMARY KEY (S#,P#)
> This is simplified from an example in "An Introduction to Database
> Systems", by C.J. Date, taken from the 1981 third edition. Note how
> the named domains become the default types for columns of the same
> name as the domains, while the QTY column in the SP relation has an
> explicit data type. Note also the constraints: the "PRIMARY KEY"
> statements in the RELATION definitions make uniqueness constraints,
> and the word "PRIMARY" in the DOMAIN definitions for S# and P# specify
> that these domains are foreign keys, thus demanding referential
> integrity from the SP table to the S and P tables. Neat, innit? :-)
> Does modern SQL have this stuff? I'm not up-to-date, I'm afraid...
> Popularity is the hallmark of mediocrity. --Niles Crane, "Frasier"
> Official WWW Site: http://www.postgresql.org
> Online Docs & FAQ: http://www.postgresql.org/docs
> Searchable Lists: http://www.postgresql.org/mhonarc
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