1. -- 更新数据 把link_id 为null 更改为2
    2. UPDATE customer set link_id = 2 WHERE link_id is NULL 
    4. -- 把地址为上海的更改link_id 为2 
    5. UPDATE  customer  set  link_id =2 WHERE address ='上海'
    7. -- 把年龄小于20 并且 link_id =1的增加10age
    8. UPDATE  customer  set age = age + 10 WHERE  age  < 20 AND  link_id = 1
    11. -- 删除 customer表中 link_id 为空的
    12. DELETE  FROM  customer  WHERE link_id is NULL 
    14. -- 删除最后两条数据
    15. DELETE  FROM  customer  order by id  DESC  limit 2
    18. -- 插入一条记录
    19. insert into customer  set cname='c8', age = 23, address = '上海', link_id = 3
    21. -- 插入多条记录
    22. INSERT  INTO  customer  (cname , age , address, link_id) values ('c9',24,'浦东',3),('c10',25,'pussy',3),('c11',26,'江里',3)