diary constraints

Hi folks

I know this has been discussed in the past, but no amount of keywords 
has returned anything from the archives.

I want to create a courtesy car diary diary system where I have a table 
containing all of the cortesy cars in the pool, and then an allocation 
table which has two timestamps, one for the start date/time and one for 
the return date/time.

How do I go about creating constraints on inserts and updates to ensure 
that

a) the finish is after the start
b) two allocations for a single vehicle don't overlap.
-- 
Gary Stainburn
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