面试笔试常考的mysql 数据库操作group by，mysqlgroup
IT 面试中，数据库的相关问题基本上属于必考问题，而其中关于sql语句也是经常考察的一个重要知识点。
下面介绍下sql语句中一个比较重要的操作group by，他的重要行一方面体现在他的理解困难度，一方面体现应用中的长见性。
首先，给出一个studnet学生表：
CREATE TABLE `student` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(30) DEFAULT NULL, `sex` tinyint(1) DEFAULT '0', `score` int(10) NOT NULL, `dept` varchar(10) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=MyISAM AUTO_INCREMENT=8 DEFAULT CHARSET=utf8
添加一些测试数据：
mysql> select * from student where id<10; +----+------+------+-------+---------+ | id | name | sex | score | dept | +----+------+------+-------+---------+ | 1 | a | 1 | 90 | dev | | 2 | b | 1 | 90 | dev | | 3 | b | 0 | 88 | design | | 4 | c | 0 | 60 | sales | | 5 | c | 0 | 89 | sales | | 6 | d | 1 | 100 | product | +----+------+------+-------+---------+
给出需求，写出sql：
给出各个部门最高学生的分数。
要想得到各个部门学生，首先就要分组，按照部门把他们分组，然后在各个部门中找到分数最高的就可以了。
所以sql语句为：
mysql> select *, max(score) as max from student group by dept order by name; +----+------+------+-------+---------+------+ | id | name | sex | score | dept | max | +----+------+------+-------+---------+------+ | 1 | a | 1 | 90 | dev | 90 | | 3 | b | 0 | 88 | design | 88 | | 4 | c | 0 | 60 | sales | 89 | | 6 | d | 1 | 100 | product | 100 | +----+------+------+-------+---------+------+ 4 rows in set (0.00 sec)
这只是个简单的例子，我们可以再把这个例子复杂化，比如分数最高的必须是女生，即sex列值必须为1才挑选出，这时的sql语句应该为：
mysql> select *,max(score) as max from student group by dept having sex='1' order by name; +----+------+------+-------+---------+------+ | id | name | sex | score | dept | max | +----+------+------+-------+---------+------+ | 1 | a | 1 | 90 | dev | 90 | | 6 | d | 1 | 100 | product | 100 | +----+------+------+-------+---------+------+ 2 rows in set (0.46 sec)
mysql> select *,max(score) as max from student where sex='1' group by dept order by name; +----+------+------+-------+---------+------+ | id | name | sex | score | dept | max | +----+------+------+-------+---------+------+ | 1 | a | 1 | 90 | dev | 90 | | 6 | d | 1 | 100 | product | 100 | +----+------+------+-------+---------+------+ 2 rows in set (0.05 sec)
查询出的结果时一致的，如果把选择条件改为必须部门所有人的分数之和大于150才能把分数最高的部门的人列出来，这里就必须使用having了，因为 having 里面可以使用聚合函数sum，并且也必须分完组我们才能得到这个组的总分数，才能比较是否该值大于150：
mysql> select *,max(score) as max from student group by dept having sum(score)>150 order by name; +----+------+------+-------+---------+------+ | id | name | sex | score | dept | max | +----+------+------+-------+---------+------+ | 1 | a | 1 | 90 | dev | 90 | | 6 | d | 1 | 100 | product | 100 | +----+------+------+-------+---------+------+ 2 rows in set (0.00 sec)
额外增加一个例子，比如我要选出不重复的部门，我们可以使用
mysql> select distinct dept from student; +---------+ | dept | +---------+ | dev | | design | | sales | | product | +---------+ 4 rows in set (0.02 sec)
但是如果我们还要列出他的id等一些其他信息，我们如果这样：
mysql> select name,distinct dept from student; ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'distinct dept from student' at line 1
这是不行的，因为distinct只能放到开始位置，如果：
mysql> select distinct dept,name from student; +---------+------+ | dept | name | +---------+------+ | dev | a | | dev | b | | design | b | | sales | c | | product | d | | product | m | +---------+------+ 6 rows in set (0.00 sec)
mysql> select dept,name from student group by dept; +---------+------+ | dept | name | +---------+------+ | design | b | | dev | a | | product | d | | sales | c | +---------+------+ 4 rows in set (0.00 sec)
按照dept分组，自然就达到去重的目的了。所以有时候如果我们碰到了一个问题很难解决，比如用distinct去重，并带上其他列值，我们就需要尝试换个思路，可能答案自然就找到了。
对ID和价格分组
SELECT pid, price, sum(num) as total
from A group by pid,price
order by id
首先你要在项目中加载对应数据库的驱动包，然后进行数据库查询，代码待会给你。
String sql="";
Class.forName("com.mysql.jdbc.Driver");
Connection con=DriverManager.getConnection("jdbc.mysql//localhost:3306/<你的数据库名>","username","pwd");
PreparedStatement pstmt=con.prepareStatement(sql);
ResultSet rs=pstmt.executeQuery();
while(rs.next()){
System.out.println(rs.getInt(1)+" "+rs.getInt(2));
}