add a constraint to specify NOT NULL values

Hi all;

How could I add a constraint to specify that a column can't be null? I've
tried a lot of sintaxis of the style of:

(the table is "pluviometria", and the column is "year")

ALTER TABLE pluviometria ALTER year CONSTRAINT NOT NULL;
ALTER TABLE pluviometria.year CONSTRAINT NOT NULL;
ALTER pluviometria.year ADD CONSTRAINT NOT NULL;
ALTER TABLE pluviometria ALTER COLUMN cod_variable SET NOT NULL;
...
but none works.

Best wishes

Javier

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