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UPDATE real_table R
SET (col1, col2, col3,...)
...
    FROM scratch S
    WHERE R.key_val1 = S.key_val1 AND R.key_val2 = S.key_val2 AND ...
;

INSERT INTO real_table R
SELECT * FROM scratch S
...
  SELECT 1 FROM real_table
  WHERE key_val1 = S.key_val1 AND key_val2 = S.key_val2 AND ...
)