timespan and cast

I would like to know how to implement a field of type timespan where time
ranges could be specified like from year-year. E .g if there is a field
value like 1998-2005 there should be a match with a

  select timerange from table where timerange=2000;

I would appreciate any examples.

My second problem is the cast operator. I wasn't able to set an attribute
of type oid with a value from an attribute of type int4. In the sql manpage
"CAST 'string' as new_type" is mentioned but not how to apply this within an
sql command like update.

TIA, Werner
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