SQL query: find each item that costs more than the average and how much more -

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i'm doing revision databases exam , 1 of questions follows:

given table items (columns: itemid, description, unitcost), formulate query find each item costs more average , how more average costs.

my answer far 

select itemid,         description,         unitcost - avg(unitcost)  items  unitcost > (select avg(unitcost) items)
  • would work? understand, expressions allowed in list of selected columns i'm not sure if extends use of functions avg().
  • is there cleaner, clearer way express same thing? preferably using iso sql constructs because course rather theoretical , not connected specific dialect.

thanks friends :)

one query should work version of sql (that accepts explicit join syntax) be:

select i.itemid,         i.description,         i.unitcost - a.avg_cost cost_diff (select avg(unitcost) avg_cost items) join items on i.unitcost > a.avg_cost

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