USE test;

SELECT s.id, s.name, s.class_id, c.name class_name, s.gender, s.score 

FROM students s

LEFT OUTER JOIN classes c

ON s.class_id = c.id

UNION

SELECT s.id, s.name, s.class_id, c.name class_name, s.gender, s.score 

FROM students s

RIGHT OUTER JOIN classes c

ON s.class_id = c.id;

# id, name, class_id, class_name, gender, score

1, 小明, 1, 一班, M, 90

2, 小红, 1, 一班, F, 95

3, 小军, 1, 一班, M, 88

4, 小米, 1, 一班, F, 73

5, 小白, 2, 二班, F, 81

6, 小兵, 2, 二班, M, 55

7, 小林, 2, 二班, M, 85

8, 小新, 3, 三班, F, 91

9, 小王, 3, 三班, M, 89

10, 小丽, 3, 三班, F, 85

11, 新生, 5, , M, 88

, , , 四班, ,