Noob Q collapsing multiple rows into one via case and max

[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.

kt

ps. Is this a crazy way to achieve our requirement, which is to see the
DB as it was at a point in the past? Our data is incredibly stable, btw. kt