I'm sorry but I think you're all making this more complicated than it needs
to be.
I could easily write a Java program that obtained a result set, then
processed that set in almost any way you wanted, including two records at a
time. I could probably do the same in Perl or any other language supported
by MySQL, although it might take a bit of time to learn the necessary parts
of the language. Processing a result set is a very well-understood process
and has been widely done millions of times in dozens of languages. You
should not need to add any columns to the table to do it either.
I might be able to offer more detail - and maybe even an example! - if a few
things were explained to me. I'm still not clear on why two records are
being processed at a time and what the relationship is between the records.
About the only time I see twinned records are when someone is doing an
accounting application where each debit is matched by an offsetting credit.
But this doesn't seem to the case here. Perhaps you don't need to process
two records at once?
Also, am I right in assuming that you are allowing new records to be written
to the table - and allowing existing records to be updated - while you run
your query? If yes, is it possible that the keys of the new records will be
interspersed with the existing keys? Or will new records always have a key
value that is higher than the highest previous key value? If inserts and
updates are happening, do they absolutely HAVE to happen simultaneously with
your query? Or could you store them off to the side briefly or even just
suspend the insert and update applications while the query runs?
If there are no inserts and updates to worry about, you should be able to
avoid any updates of duplicates by simply sorting all of the desired rows
into order based on the key and then processing them one (or two) at a time.
Things get a bit trickier if the table is being updated/inserted while your
new program is running.
I don't want to say much more until you've clarified what it is you are
doing....
--
Rhino
----- Original Message -----
From: "George Law" <[EMAIL PROTECTED]>
To: "Steffan A. Cline" <[EMAIL PROTECTED]>; <mysql@lists.mysql.com>
Sent: Friday, May 12, 2006 3:33 PM
Subject: RE: Returning records in a circle
I ran into this same type of question the other evening at a local
linux group.
I think that once you reach the end of the results set the only way to
start back at the beginning of that results set is to do the query
again.
Once option - do your query and grab all the rows
load them into a data structure - ie - a list of associative arrays
Then all you need to do is incrementally go through the array.
when you hit the end, just reset the index back to 0.
-----Original Message-----
From: Steffan A. Cline [mailto:[EMAIL PROTECTED]
Sent: Friday, May 12, 2006 2:52 PM
To: mysql@lists.mysql.com
Subject: Re: Returning records in a circle
Well, basically it can be done to an extent in some kind of code.
Basically
I am drawing from a table 2 records at a time. I want to make sure that
all
records are pulled at least once. If at all possible not to have 2 from
the
same vendor.
So, in this case.
I have columns id, html, vendor
So I can add columns as needed. I tried adding a flag and after
returning to
the client the 2 records I'd mark it as flag = 1 then search like this
Select id, html from urls order by flag, id desc limit 3
Then after I have those I would then set the last of the 3 to flag = 1
so
that on the next search I get the 2 after. In theory it worked fine but
when
multiple people hit the page simultaneously I had flags in different
places
and not in order. Maybe just mark them as flag = 1 after returned and
then
on search if found is 0 then set all to flag = 0 so they can be seen
again?
This doesn't seem so bad but them I guess I'd use distinct? If I cant
use
distinct with other parameters...
ex: select id,html distinct(vendor) from urls where flag=0 limit 2;
Would it be like :
Select id,html from urls where flag = 0 group by distinct(vendor) limit
2
?
Thanks
Steffan
---------------------------------------------------------------
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Steffan A. Cline
[EMAIL PROTECTED] Phoenix, Az
http://www.ExecuChoice.net USA
AIM : SteffanC ICQ : 57234309
The Executive's Choice in Lasso driven Internet Applications
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---------------------------------------------------------------
From: Rhino <[EMAIL PROTECTED]>
Date: Fri, 12 May 2006 14:20:10 -0400
To: "Steffan A. Cline" <[EMAIL PROTECTED]>, <mysql@lists.mysql.com>
Subject: Re: Returning records in a circle
----- Original Message -----
From: "Steffan A. Cline" <[EMAIL PROTECTED]>
To: <mysql@lists.mysql.com>
Sent: Friday, May 12, 2006 1:38 PM
Subject: Returning records in a circle
Is there a way I can get a set of records incrementally such as to
get 2
then the next query get the next 2 then at the end of all records to
get
the
2 from the beginning? I need to keep going incrementally by 2 in a
circle.
Are you trying to get these rows purely via SQL at the command line or
in an
SQL script? Or would an application be an option for you?
If you are not willing to consider application code to grab the rows
you
want, the answer to your question is "maybe". SQL has always been
intended
to return ALL of the rows that satisfy a query with a single
invocation of
the query, no matter how many rows that is. So if your query says:
select * from mytab;
you will normally get all of the rows that satisfy that query in one
go,
whether there are 0 rows, 100 rows, or a 100 million rows in the
result.
You _might_ be able to get the results you want by using the LIMIT
clause.
I'm not sure what version of MySQL you are using but the LIMIT clause
is
described in the MySQL 3.23/4.0/4.1 manual on this page:
http://dev.mysql.com/doc/refman/4.1/en/select.html. The problem is
that
you'll still pretty much need some sort of script in order to keep
executing
the query to get the next two records and you may need to change the
parameters of the LIMIT clause at the same time.
If you are willing to write application code, things get a lot easier.
For
instance, a Java program could easily grab rows from a result set for
you
two at a time, let you process them, then grab two more, etc. I expect
that
it would similarly easy to do the same thing in Perl and PHP and C.
In short, a program gives you a lot more ability to do what you want
to do
with your database data. But some shops have very little programming
expertise and prefer to do everything via SQL. If you work for one of
those
shops, you might not be able to get your records two at a time with
SQL
alone, unless you can write a script that takes advantage of the LIMIT
clause.
I don't pretend to know MySQL exhaustively so someone else may have
another
suggestion for you but the only two approaches I can think of that
might
meet your needs are to use the LIMIT clause or to write an
application.
--
Rhino
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