mysql 基本操作练习

 create table employee (name varchar(30), sex char(2), age int, address varchar(30));

 insert into employee values ('张三', '女', 19, '北京');
 insert into employee values ('李四', '男', 20, '上海');
 insert into employee values ('王五', '女', 25, '广州');
 insert into employee values ('薛六', '女', 20, '北京');
 insert into employee values ('王五', '男', 22, '北京');
 insert into employee values ('赵七', '男', 28, '上海');
 insert into employee values ('张四', '女', 23, '北京');

 #（1）. 写出sql语句，查询所有年龄大于20岁的员工（2分）
 select *from employee where age>20;
 #（2）. 写出sql语句，查询所有年龄小于25岁的女性员工（3分）
 select *from employee where sex='女' and age<25;
 #（3）. 写出sql语句，统计男女员工各有多少名（3分）
 #count 函数
 select count(*) from employee;#统计表元素个数
 select count(*) from employee where sex='女';#统计女生个数
 select sex,count(*) from employee group by sex;#group by sex 按性别分组，用性别标记分组
 select sex,count(*) as num from employee group by sex;#as 起别名

 #（4）. 写出sql语句，按照年龄倒序获取员工信息（3分）
 select * from employee order by age desc;
 #（5）. 写出sql语句，获取员工中哪个姓名具有重名现象（3分）
 SELECT *
   FROM employee
  WHERE name IN (SELECT name
                   FROM employee
                  GROUP BY name
                 HAVING COUNT(*) > 1)
 #（6）. 写出sql语句，查询所有姓张的员工（3分）
 select * from employee where name like '%张%';#模糊查询，只要含张字
 select * from employee where name like '张%';#只要姓张
 #占位符写法
 select * from employee where name like '张__';
 #（7）. 写出sql语句，查询住址为北京的前3条记录（3分）
 select * from employee where address='北京'  order by name asc limit 0,3;#从下标几开始 ，几个
 select * from employee limit 3,3;
 #（8）. 写出sql语句，查询员工总数（3分）
 select count(*) as allnum from employee;
 #（9）. 写出sql语句，向表中插入一条记录（2分）
 insert into employee(name,sex,age,address) values('七七','男',20,'深圳');
 #（10）.写出sql语句，修改员工张四的住址为南京（2分）
 update employee set address='南京' where name='张四';
 #（11）.写出sql语句，删除年龄大于24岁的女员工（2分）
 delete from employee where age>24 and sex='女';