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From: cortesi@infmx.UUCP (David Cortesi)
Newsgroups: comp.databases
Subject: Re: Union and Views
Message-ID: <4937@infmx.UUCP>
Date: 1 Aug 90 22:45:37 GMT
References: <27663@netnews.upenn.edu>
Reply-To: cortesi@infmx.UUCP (David Cortesi)
Distribution: usa
Organization: Informix, Menlo Park, Ca. U.S.A.
Lines: 39

In article <27663@netnews.upenn.edu> aaron@grad2.cis.upenn.edu.UUCP (Aaron Watters) writes:
>
>In Informix, for example, Unions are not allowed in the
>definitions of Views.  Why is this?
>
>Do other systems have this restriction?

Indeed they do.  First, from An Introduction To Database Systems
by Date (4th ed.1987), Section 8.2 "View Definition,"

	In principle, any derivable table -- i.e. any table that can
	be retrieved via a SELECT statement -- can theoretically be
	defined as a view.  In practice ... DB2 does not allow a
	view definition to include the UNION operator; however there
	is no intrinsic reason for that restriction, it is merely
	a quirk of DB2 per se...

Later, Date wrote The SQL Standard (2nd ed., 1989) in which he cites
the ANSI syntax for a view (page 88):

	CREATE VIEW view [ ( column-commalist) ]
	   AS query-specification
	       [WITH CHECK OPTION]

and then on page 95 he explicates the meaning of that nonterminal,
the "query-specification":

		query-expression
		  ::=   query-term
		      | query-expression UNION [ALL] query-term
		query-term
		  ::= query-specification | ( query-expression)

	The construct "query expression" appears only within a
	cursor definition (note, therefore, that UNION -- with or
	without ALL -- is not allowed in ... a view definition).

So it would appear that the "quirk" of DB2 has been enshrined in
the ANSI standard.  Hope you find this informative.