面试笔试常考的mysql 数据库操作group by .

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：

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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

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：

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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 |
+----+------+------+-------+---------+

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：

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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)

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：

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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 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)

这里我们没有用where语句而是用了having,这里简单说明一下，因为我们的条件是在分组后进行的，其实分组前挑选出sex='1',然后再按照dept部门分组，也是可行的，这里就要看题目是怎么要求的：

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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)

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：

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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)

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

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mysql> select distinct dept from student;
+---------+
| dept |
+---------+
| dev |
| design |
| sales |
| product |
+---------+
4 rows in set (0.02 sec)

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：

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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

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：

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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 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)

为什么没有达到预期的效果，因为distinct 作用到了2个字段上，这时，我们就需要groub by 出场了。

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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)

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。