help with simple SQL update + join -

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i think should pretty simple, i'm sql newb.

i have 2 tables. 1 list of items ids , descriptions, other map of corresponding old , new ids. this:

    id_map old_id   new_id --------------- 1        101 2        102      items id       description -------------------- 1        "itema" 2        "itemb" ... 101      <null> 102      <null>

i need copy old item descriptions new items according map. think need use inner join inside of update, it's not working , i'm not sure that's right way go.

i'm trying statements like

update items set (select items.description      items      join id_map      on items.id = id_map.new_id) =      (select items.description      items      join id_map      on items.id = id_map.old_id)

but of course it's not working. should doing?

update new_item     set description = old_item.description     items old_item         inner join id_map im             on old_item.id = im.old_id         inner join items new_item             on im.new_id = new_item.id

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