内部联接在null条件：bug或功能？

Here is the test setup:

这是测试设置:

CREATE TABLE A (
    id bigint NOT NULL AUTO_INCREMENT,
    value bigint,
    PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
INSERT INTO A (id, value) VALUES (1, 22);
INSERT INTO A (id, value) VALUES (2, 25);
INSERT INTO A (id, value) VALUES (3, 25);

CREATE TABLE B (
    id bigint NOT NULL AUTO_INCREMENT, 
    value bigint, 
    PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
CREATE TABL