mysql主键操作
删除表主键：
alter table student drop primary key;
增加表主键：
alter table student add primary key(id);
重新设置主键自增(前提是需要先执行删除表主键语句)：
alter table student modify id bigint(20) auto_increment primary key;
验证是否重名
加上limit 1 就是告诉mysql 找到即刻返回，不浪费后面的时间
select count(*) from student where name = 'tom' limit 1;
group by语句：按条件分成不同的组
新建customer表
| id | name | cost | |
| 1 | tom | 1000 | |
| 2 | jojnny | 2000 | |
| 3 | tom | 350 | |
| 4 | bruce | 730 | |
| 5 | mark | 3000 | |
| 6 | ethan | 2300 | |
| 7 | bruce | 1740 |
查询每个客户花费的总额：
select name, sum(cost) from customer group by customer;
查询的结果为：
| name | sum(cost) |
| bruce | 2470 |
| ethan | 2300 |
| johnny | 2000 |
| mark | 3000 |
| tom | 1350 |
order by语句：按条件顺序排列
按花费从小到大查询
select * from customer order by cost;
查询结果为
| id | customer | cost |
| 3 | tom | 350 |
| 4 | bruce | 730 |
| 1 | tom | 1000 |
| 7 | bruce | 1740 |
| 2 | johnny | 2000 |
| 6 | ethan | 2300 |
| 5 | mark | 3000 |
group by order by组合：按分组条件分组后，再按排序条件排序
按客户总花费从小到大排列
select name, sum(cost) from customer group by customer order by sum(cost);
查询结果为：
| name | sum(cost) |
| tom | 1350 |
| johnny | 2000 |
| ethan | 2300 |
| bruce | 2470 |
| mark | 3000 |
distinct语句:去除重复项
查询出所有的客户名字
select distinct name from customer;
查询结果为：
| name |
| tom |
| johnny |
| bruce |
| mark |
| ethan |