HiveBrain v1.2.0
Get Started
← Back to all entries
gotchasqlModerate

What's the difference between fetch and fetch next?

Submitted by: @import:stackexchange-dba··
0
Viewed 0 times
thewhatfetchnextdifferencebetweenand

Problem

I've got a cursor sending out pager messages, and occasionally it sends out duplicates. The syntax looks like this:

declare @col1 varchar(255),
    @col2 varchar(255)

    declare c cursor local for

    select column1, column2
    from table
    where flippity = 'floppity'

    open c
    fetch  from c into @col1, @col2
    While @@FETCH_STATUS = 0
    begin

    sp_do_your_thing @col1, @col2

    fetch next from c into @col1, @col2
    end
    close c
    deallocate c


Question: if I'm getting duplicates, could it be the fetches? do they both need to be fetch next, or neither?

Solution

There is no difference, and omitting NEXT will not lead to duplicates or different results in any way. NEXT is just an explicit declaration of the default behavior. From the syntax diagram:

FETCH   
      [ [ NEXT | PRIOR | FIRST | LAST   
                | ABSOLUTE { n | @nvar }   
                | RELATIVE { n | @nvar }   
           ]   
           FROM


NEXT is not the only option, and most of the other options are only available with certain types of cursors. It's hard for me to get into this habit, because I exclusively use fast-forward, single-pass, "firehose" cursors, but best practice is to include NEXT explicitly.

I would look at the query, as I suspect it will also be returning duplicates when there is no cursor involved. Or, whatever the stored procedure is doing, is making it seem like there are duplicates. Another possible explanation is if the query is using WITH (NOLOCK) / READUNCOMMITTED and you are reading the same row twice in an allocation order scan due to data movement.

Code Snippets

FETCH   
      [ [ NEXT | PRIOR | FIRST | LAST   
                | ABSOLUTE { n | @nvar }   
                | RELATIVE { n | @nvar }   
           ]   
           FROM

Context

StackExchange Database Administrators Q#204712, answer score: 11

Revisions (0)

No revisions yet.