--分页

-- limit start ，count

-- 限制查询出来的数据个数 

select * from students limit 5； //查询出来的数据显示五个

-- 查询前5个数据 

select * from students limit 0，5  ；

--查询 ID 6 -10

select * from students limit 5，5

-- 每夜显示2个，显示第6页的信息，按照年龄从小到大排序

-- 失败 select * from students limit 2*(6-1),2;

-- 失败 select * from students limit 10,2 order by age asc

select * from students limit  order by age asc 10,2

select * from students where gender = 2 order by desc limit 0,2

--连接查询

-- 内连接查询：

--inner join 

-- select * from A inner join B；

select * from students inner join classes； // 把 classes 和 students  合起来

-- 查询 有能够对应班级的学生以及班级信息

select *  from students inner join classes on students.cls_id = class.id

-- 按照要求显示班级和名称

-- select students.*,classes.* from students inner join classes on students.cls_id = classes.id

-- select students.name,classes.name from students inner join classes on students.cls_id = classes.id

-- 给表起名字

 select s.name,c.name from students as s inner join classes as c on s.cls_id = c.id

--显示所有 学生信息 加上班级名称

 select s.*,c.name from students as s inner join classes as c on s.cls_id = c.id

-- 查询 又能够对应班级的学生以及班级信息，按照升序排

 select s.*,c.name from students as s inner join classes as c on s.cls_id = c.id order by c.name，s.id

--外连接

-- 左连接

--left join

-- 查询每位学生对应的班级信息 在left join 左边的 作为基准 有就显示 没有就显示 null

select * from students as s left join classes as c on c.cls_id = c.id

-- right join