对数据库知识的回顾

首先，我们需要准备几张表，并适当放点数据进去

my_class表

my_day表

my_school表

my_subject

my_schedule表

1.取出一张表中所有的字段

select * from my_schedule 

2.取出一张表中特定字段

select dex from my_schedule

3..取出一张表中所有数据并以某个字段升序、降序排序

select * from my_schedule as c order by c.dex, c.day asc

4.左连接

select c.id,c.dex,d.subjectname,f.dayname,b.class_name,a.schoolname from my_schedule as c 
left join my_class as b 
on c.classid=b.classid 
left join my_school as a
on a.schoolid=c.schoolid
left join my_subject as d
on d.subjectid=c.subjectid
left join my_day as f
on f.dayid=c.day
order by c.dex, c.day asc

5.左连接后的特定数据

select c.id,c.dex,d.subjectname,f.dayname,b.class_name,a.schoolname from my_schedule as c 
left join my_class as b 
on c.classid=b.classid 
left join my_school as a
on a.schoolid=c.schoolid
left join my_subject as d
on d.subjectid=c.subjectid
left join my_day as f
on f.dayid=c.day
where c.dex=1 and c.subjectid=1
order by c.dex, c.day asc