查询条件
(1)简单查询
select * from Info
select Code as '代号',Name as '姓名' from Info
- 条件查询
Where后面跟条件 条件要写清楚
查询成绩表中成绩（degree）为92的信息
Select * from score where degree =”92”;
查询成绩表中课程号是3-245并且成绩是86的信息
Select * from score where cno='3-245' and degree=”86”
或者用or 并且用and
- 模糊查询 like not like
查找老师表中姓李的 名字是两个字老师
select * from teacher
where tName like '%李%'
%代表任意多个字符 _代表一个字符
(4)排序查询 order by 字段 排序值（desc/asc ）
select * from student order by class asc
(5)范围查询 关系运算符 between。。。and
select * from Car where Price>=40 and Price<=60
select * from Car where Price between 40 and 50、
(6)离散查询 in not in
select * from student where sname in ('张三','李四')
。。。where sname =“张三” or sname =“李四”
(7)聚合函数，统计查询
select sum(Price) from Car #查询所有价格之和 sum()求和
select count(Code) from Car #查询数据条数
select max(Code) from Car #求最大值
select min(Brand) from Car #求最小值
select avg(Price) from Car #求平均值
(8)分页查询 limit 从第几条开始，取多少条数据
#每页显示5条数据，取第2页的数据
select * from student limit (pageSize-1)*5,5
(9)去重查询distinct
select distinct cno from score;
(10)分组查询 group by 字段 having 条件
select count(*),cno,group_concat(degree),sum(degree) from score group by cno ;
select cno,group_concat(degree),sum(degree) from score group by cno having count(*)>3
#分组之后根据条件查询使用having 不使用where
高级查询
- 连接查询，对列的扩展
Select * from student as stu,score as sc
where stu.sno = sc.sno and sc.sno = “103” ;
2.联合查询，对行的扩展
select Code,Name from Info
union
select Code,Name from Nation
3.子查询
(1)无关子查询
外层查询 (里层查询)
子查询的结果当做父查询的条件
子查询：select Code from Nation where Name='汉族'
父查询：select * from Info where Nation = ''
select * from Info where Nation = (select Code from Nation where Name='汉族')
(2)相关子查询
查询汽车表中油耗低于该系列平均油耗的所有汽车信息
父查询：select * from Car where Oil<(该系列平均油耗)
子查询：select avg(Oil) from Car where Brand = '某个系列'
select * from Car a where Oil<(select avg(Oil) from Car b where b.Brand = a.Brand )