Noob Q collapsing multiple rows into one via case and max

On 1/6/2011 5:50 PM, Kenneth Tilton wrote:

[a meta-question for all the below is "what's a good link for hairy SQL"?]

A while ago I worked on a project where we had some hairy SQL
collapsing multiple rows of pseudo-rdf triples (columns
subject,predicate, and object) into one flattened row in which a
hard-coded case/max (I forget the exact syntax) plucked out a value
for a column "X" by looking for a row with the predicate "X". So we'd
find all the rows for one subject (a logical row ID) and collapse it
into a single row of conventional columns.

I now want to do something similar, but combining a conventional XY
table with columns X and Y and a separate change-history table with
columns row-id, column-name, value, and last-time-in-effect, to which
old values get pushed by an update trigger on XY when X or Y change.

The goal is to specify a time in the past and recreate a row of XY as
of that time, by starting from the current row and replacing X and Y
if necessary by the value that would have been current then, ie with
the max last-time-in-effect <= the specified time.

My thought was to read XY for a desired row-id with a left join to the
change-history table where last-time-in-effect <= the specified time
(pretty obvious) but the stumper is the clause for the x-then and
y-then columns. A useless start:

case when column_name = "X" then change_history.value else XY.X
end case as x-then

I had some luck with that, but I also want to pick out the most recent
change up to the target time where there were multiple changes.

btw, I have this (actual names with a "states" table and changes held in
a "triple" table modelled after rdf where s=row-id, p=column-name, and
ostr = the prior value):

select max (case when p='name' then ostr else 'n' end) as xx
from states left outer join triple on states.id=s
where s=1 and created < '2011-01-06 15:52:30';

That happens to produce the right result but only because the rows are
coming back in order created...hmm. Can I rely on that? Meaning the left
join on the "triple" table will always return them in the order created?

kt