mysql字段为空怎么查出所有数据？

Question

A表是学生表，B表是班级表，我这样写是没问题的：

但是如果我把id去掉，也就是a.class_id为空就查不出数据了。这个字段是非必填的，前端可以传也可以不传。该怎么优化呢？

Answer 1

应该是left join吧。

SELECT
a.*, b.class_name
FROM students AS a
Left JOIN class AS b
  ON a.class_id = b.class_id

Answer 2

SELECT
a.*, b.class_name
FROM students AS a
INNER JOIN class AS b
  ON a.class_id = b.class_id