Postgres 17

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CREATE TABLE X(A INT CHECK (A > 0));

CREATE TABLE

INSERT INTO X VALUES (0);

ERROR:  new row for relation "x" violates check constraint "x_a_check"
DETAIL:  Failing row contains (0).

INSERT INTO X VALUES (1);

INSERT 0 1

INSERT INTO X VALUES (NULL);

INSERT 0 1

SELECT *
FROM X;

a

1

null

SELECT 2

SELECT *
FROM X
WHERE A > 0;

a

1

SELECT 1

a
1
null