[Haskelldb-users] Querying an empty table

[Gregory W. <gw...@co...>]

Hi,

I have a question about how to query an empty table. (I have to query 
the
table to find the maximum value of a field; if there are any entries at 
all the
field is non-null.)

I used

last  = do
             events <- table event
             restrict (events!sid .==. constant 1)
             u <- project (cid << _max(events!cid))
             return u


The fields are sid and cid, and the goal is to find the largest value
of cid. Since cid is non-null as a type, this gives an error if the 
table
is empty.

If there are any entries at all in the table, there are over 100,000,
so I don't want to return the whole table just to see if it is 
non-empty.

My question: is there a way to simply have the query return empty
list if the table is empty, in a way that typechecks, or

is it just as efficient to do a lazy query (no "project"), see if the 
head
is non-empty and then repeat the query with the projection? Or is
there some better way of structuring this that I haven't learned yet?

By the way, the previous advice about using _default for auto-increment
fields works perfectly with PostgreSQL v. 7.4.2. Thanks for the hint. 
I'm
slowly getting better at finding examples in the test programs but
I'm still very much an HDB newbie.

Best Wishes,

Greg

Gregory Wright
Antiope Assoicates LLC
gw...@an...