Rule on tabel

Hi their,
I want to create a rule, whenever a specific row in a table is
increased, another row is incremented.

Let's say, I have a table like this:
CREATE TABLE "test" (
  "Code" VARCHAR(20) NOT NULL,
  "Number" INTEGER NOT NULL,
  "increase" INTEGER NOT NULL,
  PRIMARY KEY("Code")
)

So, I want to have a rule (or trigger or whatever) which does the
following job:

Whenever the Number filed is updated and the new value is bigger than
the old once, the increase field should be incremented (increase++).

Can anybody tell me how to do so ?

Thanks

Markus

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