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面试笔试常考的mysql数据库操作groupby.

时间：2021-07-01 10:21:17 帮助过：20人阅读

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： [s

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：

[sql] view plaincopyprint?

  1. CREATE TABLE `student` (
  2. `id` int(11) NOT NULL AUTO_INCREMENT,
  3. `name` varchar(30) DEFAULT NULL,
  4. `sex` tinyint(1) DEFAULT '0',
  5. `score` int(10) NOT NULL,
  6. `dept` varchar(10) DEFAULT NULL,
  7. PRIMARY KEY (`id`)
  8. ) 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：


[sql] view plaincopyprint?

  1. mysql> select * from student where id<10;
  2. +----+------+------+-------+---------+
  3. | id | name | sex | score | dept |
  4. +----+------+------+-------+---------+
  5. | 1 | a | 1 | 90 | dev |
  6. | 2 | b | 1 | 90 | dev |
  7. | 3 | b | 0 | 88 | design |
  8. | 4 | c | 0 | 60 | sales |
  9. | 5 | c | 0 | 89 | sales |
  10. | 6 | d | 1 | 100 | product |
  11. +----+------+------+-------+---------+ 

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：

[sql] view plaincopyprint?

  1. mysql> select *, max(score) as max from student group by dept order by name;
  2. +----+------+------+-------+---------+------+
  3. | id | name | sex | score | dept | max |
  4. +----+------+------+-------+---------+------+
  5. | 1 | a | 1 | 90 | dev | 90 |
  6. | 3 | b | 0 | 88 | design | 88 |
  7. | 4 | c | 0 | 60 | sales | 89 |
  8. | 6 | d | 1 | 100 | product | 100 |
  9. +----+------+------+-------+---------+------+
  10. 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：

[sql] view plaincopyprint?

  1. mysql> select *,max(score) as max from student group by dept having sex='1' order by name;
  2. +----+------+------+-------+---------+------+
  3. | id | name | sex | score | dept | max |
  4. +----+------+------+-------+---------+------+
  5. | 1 | a | 1 | 90 | dev | 90 |
  6. | 6 | d | 1 | 100 | product | 100 |
  7. +----+------+------+-------+---------+------+
  8. 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：

[sql] view plaincopyprint?

  1. mysql> select *,max(score) as max from student where sex='1' group by dept order by name;
  2. +----+------+------+-------+---------+------+
  3. | id | name | sex | score | dept | max |
  4. +----+------+------+-------+---------+------+
  5. | 1 | a | 1 | 90 | dev | 90 |
  6. | 6 | d | 1 | 100 | product | 100 |
  7. +----+------+------+-------+---------+------+
  8. 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：

[sql] view plaincopyprint?

  1. mysql> select *,max(score) as max from student group by dept having sum(score)>150 order by name;
  2. +----+------+------+-------+---------+------+
  3. | id | name | sex | score | dept | max |
  4. +----+------+------+-------+---------+------+
  5. | 1 | a | 1 | 90 | dev | 90 |
  6. | 6 | d | 1 | 100 | product | 100 |
  7. +----+------+------+-------+---------+------+
  8. 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

[sql] view plaincopyprint?

  1. mysql> select distinct dept from student;
  2. +---------+
  3. | dept |
  4. +---------+
  5. | dev |
  6. | design |
  7. | sales |
  8. | product |
  9. +---------+
  10. 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：

[sql] view plaincopyprint?

  1. mysql> select name,distinct dept from student;
  2. 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：

[sql] view plaincopyprint?

  1. mysql> select distinct dept,name from student;
  2. +---------+------+
  3. | dept | name |
  4. +---------+------+
  5. | dev | a |
  6. | dev | b |
  7. | design | b |
  8. | sales | c |
  9. | product | d |
  10. | product | m |
  11. +---------+------+
  12. 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。


[sql] view plaincopyprint?

  1. mysql> select dept,name from student group by dept;
  2. +---------+------+
  3. | dept | name |
  4. +---------+------+
  5. | design | b |
  6. | dev | a |
  7. | product | d |
  8. | sales | c |
  9. +---------+------+
  10. 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。

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