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单表查询
①查询所有 *
mysql> select * from student;
②查询选中字段记录
mysql> select s_name from student;
③条件查询 where
mysql> select s_name from student where s_id<5;
④查询后为字段重命名 as
mysql> select s_name as 名字 from student;
其中as可以省略不写select s_name 名字 from student;
⑤模糊查询 like
%匹配多个字符
mysql> select s_name as 姓名 from student where s_name like '李%';
_匹配一个字符
mysql> select s_name as 姓名 from student where s_name like '李_';
mysql> select s_name as 姓名 from student where s_name like '李__';
⑥排序（默认升序） order by 以某个字段为主进行排序
升序 asc （asc可以不写）
mysql> select * from student order by sc_id asc;
降序 desc
mysql> select * from student order by sc_id desc;
⑦限制显示数据数量 limit
limit 只接一个数字n时表示显示前面n行
mysql> select * from student limit 5;
limit 接两个数字m，n时表示显示第m行之后的n行
mysql> select * from student limit 2,4;
⑧常用聚合函数
mysql> select * from details;
最大值 max
mysql> select max(age) from details;
最小值 min
mysql> select min(age) from details;
求和 sum
mysql> select sum(age) from details;
平均值 avg
mysql> select avg(age) from details;
四舍五入 round
mysql> select round(avg(age)) from details;
统计 count
mysql> select count(address) from details;
⑨分组查询 group by 筛选条件使用having，having后接条件必须是select后存在的字段
mysql> select age,count(age) from details group by age having age>30;
以age为组统计每个age的人数最后筛选出age大于30的
2、子查询 也叫嵌套查询
mysql> select * from details where age>(select avg(age) from details);
查询所有age大于平均年龄的信息
3、关联查询
①内连接 inner join
无条件内连接 又称笛卡尔连接
mysql> select * from student inner join college;
有条件内连接 在无条件基础上on接条件
mysql> select * from student inner join college on sc_id=c_id;
②外连接
左外连接 left join
以左表为基准，右表没有对应数据以null填充，多余数据去除
mysql> select * from tb1 left join tb2 on id=t_id;
mysql> select * from tb2 left join tb1 on id=t_id;
右外连接 right join
以右表为基准，左表没有对应数据以null填充，多余数据去除
mysql> select * from tb1 right join tb2 on id=t_id;
mysql> select * from tb2 right join tb1 on id=t_id;
派生表必须命名 as
mysql> select * from (select * from details where age>30) as a left join student on d_id=s_id;
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