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mysql字段为空怎么查出所有数据?

A表是学生表,B表是班级表,我这样写是没问题的:
image.png

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?


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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

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