查詢數據：
簡單查詢：
1.查詢所有數據：
select * from 表名
#select * from info 查詢所有列
2.查詢指定列：
select code,name from info
3.查詢指定行（條件查詢）：
select * from info where nation='n001'
select * from info where code='p001' or/and nation='n001'
4.給列制定名稱
select code as '代號' name as '姓名' from info
5.模糊查詢
select * from car where name like '%/_奧迪%' #'%'---表示在‘奧迪’前后可以出現n個字符，'_'表示在‘奧迪’前后可以出現1個字符
6.排序查詢
select * from car order by price #根據價格進行排序，默認升序（asc），后面加desc降序
select * from car order by price asc，oil desc #先根據價格升序排列，在次前提下出現價格相同的項再按油耗降序排列
7.去重查詢
select distinct brand from car
8.分頁查詢
select * from car limit 5,5 跳過前面5條取5條數據
9.統計查詢（聚合函數）
數據條數：select count(主鍵/*) from car
取最大值/最小值：select max/min(price) from car
取平均值：select avg(price) from car
10.分組查詢
select brand,count(*) from car group by brand
select brand from car group by brand having count(*)>=3
11.范圍查詢
select * from car where price>=40 and price <=60
select * from car where price between 40 and 60
12.離散查詢
select* from car where price in/not in(10,20,30,40)
高級查詢：
1.聯合查詢
select code,name from info
union
select code,name from nation
2.連接查詢
形成笛卡爾積
select * from info,nation where info.nation=nation.code
為了查詢效率更高可以篩選出重復的數據：
select info.name,info.sex,nation.name,info.birthday from info,nation where info.nation=nation.code
select info.name,info.sex,nation.name,info.birthday from info join nation on info.nation=nation.code
3.子查詢
---子查詢的結果作為父查詢的條件使用
i.無關子查詢
---子查詢和父查詢沒有關系，子查詢單獨拿出來可以執行
a.查找民族為“漢族”的所有人員信息
select * from info where nation =(select code from nation where name='漢族')
b.查詢所有廠商是“一汽大眾”的車
select * from car where brand in(select brand_code from brand where prod_code in(select prod_code from productor where prod_code='一汽大眾'))
ii.相關子查詢
查詢油耗低於該系列平均油耗的汽車信息
select * from car where oil<(該系列平均油耗)
select avg(oil) from car where brand= 該系列
select * from car a where oil<(select avg(oil) from car b where b.brand=a.brand)