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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