How to test for existence of nothing

Keith Bradnam krb at sanger.ac.uk
Thu Jul 19 04:11:18 EST 2001

Any acedb query gurus out there who can help me with a problem?

It might be that it's beyond the capabilities of the acedb query language
in which case Ed will set me straight.

How do you test for the absence of a value in a tag-value pair where you
would normally expect a value?

E.g. we have sequence objects with Homol tags.  Associated with the Homol
tag should be the name of a Sequence object.  Occasionally though, errors
creep in where we lose the object and are left with just a tag.

I would ideally like to be able to query the database to find errant
entries like this.  I thought I might be able to get away with things like

Find Sequence Homol AND NEXT = ""

But (perhaps obviously) the 'AND NEXT' can't actually step any further to
the right as there is nowhere to go.

Maybe I should be looking into the dark murky world of AQL queries?

I guess I want a perl style 'if (defined($tag))' to test for the presence 
(or absence) of a value for any specified tag.

Any ideas anyone?  This would be really useful to me if it is possible.


~  Keith Bradnam - WormBase group: http://wormbase.sanger.ac.uk/
~  The Sanger Centre, Wellcome Trust Genome Campus
~  Hinxton, Cambridge, CB10 1SA, UK.  Tel (01223) 497516

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