'or' or 'union'


I was just wondering, in general what is better to use, an 'or' clause in a
select or a 'union' to join two selects together. Do both statements create
work tables?

Many thanks


10/11/1999 2:33:26 PM
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I think OR will be better than union. because suppose u have 3 tables and using
OR u can join table a and table b and table c so each table will have only one
read. but using union
you will join table a and table b and in another query of union u will use
table (a or b) and table c
so ur one read is more in union .

Alex Cheung wrote:

> Hello
> I was just wondering, in general what is better to use, an 'or' clause in a
> select or a 'union' to join two selects together. Do both statements create
> work tables?
> Many thanks
> Alex

10/15/1999 5:32:34 PM
If you want to know if they create work tables, use:
and run the query.  There will be information in there regarding if a work table
is created.

UNION creates a work table.
OR may not.

OR may not always do multiple-starts on keyed values (sometimes best).  So, you
might want to use a UNION ALL (read docs on UNION vs. UNION ALL).  UNION ALL
just runs multiple queries and returns to combined results.  SOmething like

Using OR's
FROM sysobjects
where id in (1,2,3,4)

FROM sysobjects
where id = 1
FROM sysobjects
where id = 2
FROM sysobjects
where id = 3
FROM sysobjects
where id = 4

Look at SET STATISTICS IO ON.  Find the best query plan that uses the fewest
amount of Logical I/Os.  Don't worry as much about Work tables, but rather worry
about total Logical I/Os being done.

John McVicker
Xtend-Tech, Inc.

On Mon, 11 Oct 1999 15:33:26 +0100,
 in sybase.public.sqlserver.performance+tuning
Alex  Cheung <alex.cheung@neonsoft.com> wrote: 
>I was just wondering, in general what is better to use, an 'or' clause in a
>select or a 'union' to join two selects together. Do both statements create
>work tables?
>Many thanks
10/15/1999 5:42:16 PM

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