查找每组出现次数最多的条目

Question

我有以下（简化的）架构。

CREATE TABLE TEST_Appointment(
    Appointment_id INT AUTO_INCREMENT PRIMARY KEY,
    Property_No INT NOT NULL,
    Property_Type varchar(10) NOT NULL
);

INSERT INTO TEST_Appointment(Property_No, Property_Type) VALUES
    (1, 'House'),
    (1, 'House'),
    (1, 'House'),
    (2, 'Flat'),
    (2, 'Flat'),
    (3, 'Flat'),
    (4, 'House'),
    (5, 'House'),
    (6, 'Studio');

我正在尝试编写查询以获取每个属性类型组中预约次数最多的属性。示例输出为：

Property_No | Property_Type | Number of Appointments
-----------------------------------------------------
1           | House         | 3
2           | Flat          | 2
6           | Studio        | 1

我有以下查询来获取每个属性的约会数量，但我不确定如何从那里开始

SELECT Property_No, Property_Type, COUNT(*)
from TEST_Appointment
GROUP BY Property_Type, Property_No;

Answer 1

如果你是运行MySQL8.0，可以使用聚合和window函数：

select *
from (
    select property_no, property_type, count(*) no_appointments,
        rank() over(partition by property_type order by count(*) desc) rn
    from test_appointment
    group by property_no, property_type
) t
where rn = 1

在早期版本中，一个选项使用 having 子句和 row-limiting 相关子查询：

select property_no, property_type, count(*) no_appointments
from test_appointment t
group by property_no, property_type
having count(*) = (
    select count(*)
    from test_appointment t1
    where t1.property_type = t.property_type
    group by t1.property_no
    order by count(*) desc
    limit 1
)

请注意，如果有的话，两个查询都允许联系。

查找每组出现次数最多的条目

Finding the entry with the most occurrences per group

mysql

sql

count

aggregate-functions

greatest-n-per-group