Is Cursor Best Way To Go?

I need to get two values from a complex SQL statement which returns a single
record and use those two values to update a single record in a table. In
order to assign those two values to variables and then use those variables
in the UPDATE statement, I created a cursor and used Fetch Next... Into.
This way, I only have to call the complex SQL once instead of twice.
This seems like the best way to go. However, I've always used cursors for
scrolling through resultsets. In this case, though, there is just a single
record being returned, and the cursor doesn't scroll.
Is that the most efficient way to go, or is there a better way to be able to
use both values from the SQL statement without having to call it twice?
Thanks.Hi Neil
I'd need more details regarding the query/DDL to say anything too
meaningful, but certainly a set-based solution is always preferable to
an iterative/cursor-solution.|||here is a guess without seeing your code.
declare @.v1 int, @.v2 int
select @.v1=[col1], @.v2=[colx]
from (
-- your complex query
) as derived_table
-oj
"Neil" <nospam@.nospam.net> wrote in message
news:vhAne.4180$s64.2269@.newsread1.news.pas.earthlink.net...
>I need to get two values from a complex SQL statement which returns a
>single record and use those two values to update a single record in a
>table. In order to assign those two values to variables and then use those
>variables in the UPDATE statement, I created a cursor and used Fetch
>Next... Into. This way, I only have to call the complex SQL once instead
>of twice.
> This seems like the best way to go. However, I've always used cursors for
> scrolling through resultsets. In this case, though, there is just a single
> record being returned, and the cursor doesn't scroll.
> Is that the most efficient way to go, or is there a better way to be able
> to use both values from the SQL statement without having to call it twice?
> Thanks.
>|||SQL Server doesn't support the standard SQL syntax for this but it does
have a proprietary syntax to do the same job:
UPDATE T1
SET x = foo,
y = bar
FROM
(SELECT foo, bar /* your query here */
FROM ... ) AS T2
WHERE T2.key_col = T1.key_col
/* join condition should yield a single row from T2 for each row in
T1 */

> I've always used cursors for
> scrolling through resultsets
Really? For what purpose? Cursors should be the rare exception rather
than the rule. Usually there are better set-based solutions.
David Portas
SQL Server MVP
--|||> SQL Server doesn't support the standard SQL syntax for this but it does
> have a proprietary syntax to do the same job:
> UPDATE T1
> SET x = foo,
> y = bar
> FROM
> (SELECT foo, bar /* your query here */
> FROM ... ) AS T2
> WHERE T2.key_col = T1.key_col
> /* join condition should yield a single row from T2 for each row in
> T1 */
Yes, that was what I was looking for (though I needed to use UPDATE T1
SET... From T1, (Select foo...) As T2...)
Also, since I'm only updating a single row in T1, and since T2 only returns
a single row with values, I eliminated the WHERE T2.keycol=T1.keycol. My SQL
looks like:
UPDATE T1
SET X = T2.FOO, Y=T2.BAR
FROM T1, (SELECT FOO, BAR FROM MYQUERY WHERE ID=@.VALUE) AS T2
WHERE T1.ID=@.VALUE
Do you see any problem with that?

> Really? For what purpose? Cursors should be the rare exception rather
> than the rule. Usually there are better set-based solutions.
I guess one of the main areas where I've used them is in order-rearranging
functions -- such as where there are a set of items in a table, each with a
value in a field that specifies the order. The user clicks, say, an up arrow
in the interface, and the current item needs to move up one in order --
decrement it's field value by one, and increment the preceding item's by
one.
Another time I used a cursor was in a procedure in which the length of two
fields combined needed to be compared to a value and then, based on the
length of the combined fields, different values would be placed in a certain
field. I suppose that could have just been done with a set-based solution;
but the cursor seemed more straightforward. It was also only dealing with
one record at a time.
Thanks for your help!
Neil

> --
> David Portas
> SQL Server MVP
> --
>|||Re-arranging order based on a column (pos):
UPDATE foo
SET pos = CASE pos
WHEN @.old_pos
THEN @.new_pos
ELSE pos + SIGN(@.old_pos - @.new_pos)
END
WHERE pos BETWEEN @.old_pos AND @.new_pos
OR pos BETWEEN @.new_pos AND @.old_pos
Update different columns based on the length of a string value:
UPDATE YourTable
SET col1 =
CASE
WHEN LEN(x+y)<=10
THEN a ELSE b END,
col2 =
CASE
WHEN LEN(x+y)>10
THEN a ELSE b END
WHERE ...
David Portas
SQL Server MVP
--
UPDATE|||David Portas Jun 2, 7:07 am show options
Newsgroups: comp.databases.ms-sqlserver,
microsoft.public.sqlserver.programming
From: "David Portas" <REMOVE_BEFORE_REPLYING_dpor...@.acm.o=ADrg> - Find
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Date: 2 Jun 2005 04:07:17 -0700
Local: Thurs,Jun 2 2005 7:07 am
Subject: Re: Is Cursor Best Way To Go?
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Re-arranging order based on a column (pos):
UPDATE foo
SET pos =3D CASE pos
WHEN @.old_pos
THEN @.new_pos
ELSE pos + SIGN(@.old_pos - @.new_pos)
END
WHERE pos BETWEEN @.old_pos AND @.new_pos
OR pos BETWEEN @.new_pos AND @.old_pos;
Very neat! I always did a monster CASE expression with extra WHEN
clauses based on (old_pos ' newpos).|||Hi, David.
Here's another one for you. I have an sp that takes various input parameters
for a customer, and processes the data using various case statements. I now
want to run this sp for all customers on a nightly basis. My immediate
reaction, as previously, would be to use a cursor to loop through all the
customers, get the input parameters for the sp from the Customer table, and
call the sp once for each customer. Is there a way to do this without a
cursor?
Thanks,
Neil
"David Portas" <REMOVE_BEFORE_REPLYING_dportas@.acm.org> wrote in message
news:1117710437.665131.255080@.g47g2000cwa.googlegroups.com...
> Re-arranging order based on a column (pos):
> UPDATE foo
> SET pos = CASE pos
> WHEN @.old_pos
> THEN @.new_pos
> ELSE pos + SIGN(@.old_pos - @.new_pos)
> END
> WHERE pos BETWEEN @.old_pos AND @.new_pos
> OR pos BETWEEN @.new_pos AND @.old_pos
> Update different columns based on the length of a string value:
> UPDATE YourTable
> SET col1 =
> CASE
> WHEN LEN(x+y)<=10
> THEN a ELSE b END,
> col2 =
> CASE
> WHEN LEN(x+y)>10
> THEN a ELSE b END
> WHERE ...
> --
> David Portas
> SQL Server MVP
> --
>
>
>
> UPDATE
>|||Neil (nospam@.nospam.net) writes:
> Here's another one for you. I have an sp that takes various input
> parameters for a customer, and processes the data using various case
> statements. I now want to run this sp for all customers on a nightly
> basis. My immediate reaction, as previously, would be to use a cursor to
> loop through all the customers, get the input parameters for the sp from
> the Customer table, and call the sp once for each customer. Is there a
> way to do this without a cursor?
Yes, but you will of course have to rewrite the procedure, so that it
works with many customers. To do this, you need to pass the input
parameters in a table rather than as parameter. This table can be a temp
table, or a permanent table which is keyed by @.@.spid or similar. I discuss
this on http://www.sommarskog.se/share_data.html#temptables.
Well, rather you would write a new procedure that works with many, and
then rewrite the old procedure to be a wrapper on the new procedure.
Now, whether you actually should go this route depends. Let's say that
it takes 10 minutes to run a cursor over all customers and call the
existing procedure, and that you have plenty of time to spare in the
night. In this case, it's not likely to be worth the development effort.
Also, if you opt to use a temp table to pass the input parameters, the
procedure will be recompiled each time. This will have the net effect
that calls for single customers will now be more expensive, and could
even be performance problems, if the procedure is huge.
We actually did this exercise with a core procedure in our system, and
in our case it was really necessary. But it was a major developement task.
Our estimate was 200 hours for development, but I think the true outcome
was more than 300 hours. But that was a long procedure, on 700-800 lines
and which called several sub-procedures. The final multi-version is a
3000-line monster with no less than 43 table variables.
Erland Sommarskog, SQL Server MVP, esquel@.sommarskog.se
Books Online for SQL Server SP3 at
http://www.microsoft.com/sql/techin.../2000/books.asp|||Everything Erland has said. This is where it pays to have a good design
pattern from kick-off. For an UPDATE/INSERT/DELETE proc servicing the
UI you may typically want to pass parameters for a single row. For
procs that implement other business logic however, you should generally
design with a set-based approach in mind. Unfortunately, programmers
used to other languages too often try to encapsulate all logic in procs
that act like scalar functions - a sure route to cursor hell!
David Portas
SQL Server MVP
--