Why can I create a table with PRIMARY KEY on a nullable column?

Question

The following code creates a table without raising any errors:

CREATE TABLE test(
ID INTEGER NULL,
CONSTRAINT PK_test PRIMARY KEY(ID)
)

Not

Answer 1

If as @ErwinBrandstetter said, PRIMARY KEY is merely a combination of UNIQUE and NOT NULL, you can use an UNIQUE constraint without NOT NULL instead of PRIMARY KEY. Example:

CREATE TABLE test(
    id integer,
    CONSTRAINT test_id_key UNIQUE(id)
);

This way you can do things like:

INSERT INTO test (id) VALUES (NULL);
INSERT INTO test (id) VALUES (NULL);
INSERT INTO test (id) VALUES (NULL);