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一、排序
#根据年龄进行降序排序
select * from student order by age desc;
#根据年龄进行升序排序
select * from student order by age asc;
#给字段起别名，as可以省略
select no as 编号, name as 姓名,age as 年龄 from student order by age asc;
#查找前五条
select * from student limit 5;
#从第五行开始查找三条（第一行是0，所以5实际上是第六行）
select * from student limit 5,3;
二、模糊查询
#查询name中带张的数据
select * from student where name like '张_'
#查询name中为张XX的数据
select * from student where name like '张__'
#以张开头的数据
select * from student where name like '张%'
#以张结尾的数据
select * from student where name like '%张'
#查询年龄在20到21之间，包括20和21
select * from student where age between 20 and 21
#查询年龄为20和21的数据
select * from student where age in(20,21）
三、聚合函数
#统计一共有多少学生
select count(*) from student;
#查询年龄的平均值
select avg(age) from student;
#对年龄进行求和
select sum(age) from student;
#查询年龄的最大值
select max(age) from student
#查询年龄最小值
select limit(age) from student
#查询的结果里只能有分组的列和聚合函数
select major,count(*) from student2 group by major
#having用来设置分组查询的条件
select major,count(*) from student2 group by major having count(*)>2;
四、子查询
#根据年龄的子查询
select * from student2 where age>(select age from student where name ='欧阳丹丹')
五、多表查询
#把两个表中sno相同的记录关联到一起进行输出,内连接：要求左边的表和右边的表必须对应上
select * from student inner join sc on student.sno=sc.sno where sdept='is'
#左连接：把左边表的数据都展示出来
select * from student left join sc on student.sno=sc.sno where grade>95
#右连接：把右边表的数据都展示出来
select * from student right join sc on student.sno=sc.sno where sdept='is'
#展示student.sno=sc.sno and sdept='is'的数据
select * from student,sc where student.sno=sc.sno and sdept='is'
#分组查询
select sno,avg(grade) from sc group by sno
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