mysql - 查询显示年龄超过 18 岁且具有唯一城市的学生的学生表

Question

我在 MySQL 中的表:

mysql> select * from student;
+-----+----------+--------+------+---------+
| ano | name     | gender | age  | place   |
+-----+----------+--------+------+---------+
| 114 | ron      | m      |   18 | cbe     |
| 115 | dhruv    | m      |   18 | cbe     |
| 116 | mini     | f      |   23 | chennai |
| 117 | yash     | m      |   20 | chennai |
| 118 | aathmika | f      |   19 | delhi   |
| 119 | aadhi    | m      |    9 | pune    |
+-----+----------+--------+------+---------+

有一个问题叫做: 创建一个查询来显示学生表，其中包含年龄超过 18 岁且具有唯一性的学生 城市。

根据我的说法，所需的输出:

+-----+----------+--------+------+---------+
| ano | name     | gender | age  | place   |
+-----+----------+--------+------+---------+
| 116 | mini     | f      |   23 | chennai |
| 118 | aathmika | f      |   19 | delhi   |
+-----+----------+--------+------+---------+

Or 

+-----+----------+--------+------+---------+
| ano | name     | gender | age  | place   |
+-----+----------+--------+------+---------+
| 117 | yash     | m      |   20 | chennai |
| 118 | aathmika | f      |   19 | delhi   |
+-----+----------+--------+------+---------+

我尝试过以下方法:

mysql> select distinct place from student where age>18;
+---------+
| place   |
+---------+
| chennai |
| delhi   |
+---------+
2 rows in set (0.05 sec)

我尝试向放置字段添加唯一键以使用 cbe 删除第二条记录，但我的假设是错误的。

mysql> alter table student add constraint unique(place);
ERROR 1062 (23000): Duplicate entry 'cbe' for key 'place'

mysql> alter table student modify place char(10) unique;
ERROR 1062 (23000): Duplicate entry 'cbe' for key 'place'

mysql> alter table student change place place char(10) unique;
ERROR 1062 (23000): Duplicate entry 'cbe' for key 'place'

mysql> select place from student where age>18 group by place having count(place)
=1;
+-------+
| place |
+-------+
| delhi |
+-------+

另外，

mysql> select distinct place,name,ano,age from student where age>18;
+---------+----------+-----+------+
| place   | name     | ano | age  |
+---------+----------+-----+------+
| chennai | mini     | 116 |   23 |
| chennai | yash     | 117 |   20 |
| delhi   | aathmika | 118 |   19 |
+---------+----------+-----+------+
3 rows in set (0.00 sec)

当我在不同的地方使用多个字段时，它的独特特征就丢失了!!!

我应该对上述任何查询进行哪些更改才能获得所需的输出？？？

提前致谢。

Answer 1

Create a query to display the student table with students of age more than 18 with unique city

我对此的理解是:学生应该超过 18 岁，并且他们的位置应该只在表格中出现一次。只有一行符合此条件，即 ano 118(Aathmika 19 岁，没有其他学生住在德里)。

您可以将其表述为:

select s.*
from student s
where 
    age > 18 
    and not exists(select 1 from student s1 where s1.place = s.place and s1.ano <> s.ano)