Path: utzoo!mnetor!uunet!husc6!bloom-beacon!gatech!hao!oddjob!gargoyle!ihnp4!ihuxy!chapman
From: chapman@ihuxy.ATT.COM (Ralph Chapman)
Newsgroups: comp.databases
Subject: Re: Informix query
Message-ID: <2327@ihuxy.ATT.COM>
Date: 6 Jan 88 16:01:53 GMT
References: <3072@ihlpf.ATT.COM> <3890001@hpcmmb.HP.COM> <3216@ihlpf.ATT.COM> <60@coot.AUSTIN.LOCKHEED.COM>
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 46
Summary: SQL is set-oriented and non-procedural


In article <60@coot.AUSTIN.LOCKHEED.COM> Chris Wood writes:
> In article <3216@ihlpf.ATT.COM>, lukas@ihlpf.ATT.COM (00704a-Lukas) writes:
> > 4)	Difficulty (outside of perform) of doing that most basic of
> > 	database applications: that of cycling through the rows, adding,
> > 	deleting, modifying as you go.
>
> I would like to know how to go about this also.  I am using ORACLE
> and would like to do this.  ...
>
> Is there some underlying mathematical "relational purist" reason why such
> procedural constructs are lacking in relational implementations?  ...
>
> Why Can't relational systems do this?  Or better yet, is there a
> SQL/Relational system out there that does?

SQL is set-oriented-- There is no implicit ordering in a plain vanilla "set".
SQL insists on operating on an entire set all at once (conceptually, anyway),
not an element at a time.

This is an instance of "non-procedural" programming style-- It's supposed
to SAVE you the TROUBLE of spelling out a procedure to do what you want. :-)
(Not surprisingly, it's easier for non-programmers to learn this style than
for programmers (of 3rd and previous generation languages) to learn it.)

So you have to either:

(1) formulate an "UPDATE ... WHERE" statement that selects the appropriate
    records a priori and updates just that subset (don't forget that you can
    nest SQL statements, in case the subset can't be specified by a boolean
    expression in a WHERE clause, e.g.:
		UPDATE ...
		WHERE field IN
			( SELECT ...
			  and so on )

(2) (in Oracle) make yourself an IAF application; that will let you go
    through the records one at a time.  Oracle has a command that will make
    a quick-and-dirty IAF application for a single table.

I can't figure out why SQL DBMS vendors don't supply a browser program that
will loop through a SELECTed set, as you described.  (Maybe I'll write one
someday.)

--Ralph Chapman
  ihnp4!ihuxy!chapman