db(mysql、sqlite) -- select 详情联表查询优化

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查询章节的时候也返回课程的类型数据

表结构如下

如果用联表查询如下


select vc.id, vc.title, c.type from video_chapter as vc join course as c where vc.courseid=? group by vc.id

这时候如果把 group by vc.id去掉就会发现端倪。优化成如下。

select vc.id, vc.title, a.type from video_chapter vc ,(select type from course where id=?) a where courseid=?

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