Is a STRAIGHT_JOIN like a LEFT JOIN or like an INNER JOIN? (MySQL)

straight_join.sql

create table t (
    id int primary key,
    r_id int,
    c tinyint(1) default 0
);

create table r (
    id int primary key,
    c tinyint(1) default 0
);

insert into r values
    (1, 0),
    (2, 0),
    (3, 0),
    (4, 0),
    (5, 1),
    (6, 1),
    (7, 1),
    (8, 1);

insert into t (id, r_id) values
    (1, 1),
    (2, 2),
    (3, NULL),
    (4, 74),
    (5, 5),
    (6, 6),
    (7, NULL),
    (8, 75);

select t.id, r.c from t
    left join r on r.id = t.r_id
    where r.c = 1 OR r.id IS NULL;
+----+------+
| id | c    |
+----+------+
|  3 | NULL |
|  4 | NULL |
|  5 |    1 |
|  6 |    1 |
|  7 | NULL |
|  8 | NULL |
+----+------+

select t.id, r.c from t
    inner join r on r.id = t.r_id
    where r.c = 1 OR r.id IS NULL;
+----+------+
| id | c    |
+----+------+
|  5 |    1 |
|  6 |    1 |
+----+------+

select t.id, r.c from t
    straight_join r on r.id = t.r_id
    where r.c = 1 OR r.id IS NULL;
+----+------+
| id | c    |
+----+------+
|  5 |    1 |
|  6 |    1 |
+----+------+