暂时还不直接支持，看是否能用number+check约束的方式替换unsigned

CREATE TABLE example (
    unsigned_int NUMBER(10),
    CONSTRAINT check_unsigned CHECK (unsigned_int >= 0) -- 约束：值必须为非负数
);

INSERT INTO example (unsigned_int) VALUES (100); -- 合法
INSERT INTO example (unsigned_int) VALUES (-100); -- 不合法，违反约束