SQL筛选具有不同条件的两个聚合函数

Question

我想得到之间的区别

AVG(arr_delay_new) 
WHERE fl_date BETWEEN '2017/07/24' AND '2017/07/31'

和

AVG(arr_delay)_new 
WHERE fl_date BETWEEN '2017/07/01' AND '2017/07/23'

这个当然返回0，并且没有日期条件。

SELECT 
    airline_name,
    AVG(arr_delay_new) - AVG(arr_delay_new)
FROM 
    Flight_delays F
JOIN 
    Airlines A ON A.airline_id = F.airline_id
GROUP BY 
    airline_name

Answer 1

您可以使用条件聚合：

SELECT airline_name,
       (AVG(CASE WHEN fl_date BETWEEN '2017-07-24' and '2017-07-31' THEN arr_delay_new END) -
        AVG(CASE WHEN fl_date BETWEEN '2017-07-01' and '2017-07-23' THEN arr_delay_new END)
       ) as AVG_DIFF
FROM Flight_delays F JOIN
     Airlines A
     ON A.airline_id = F.airline_id
GROUP BY airline_name;

这假设arr_delay_new具有可以平均的类型。一些数据库不愿直接对日期/时间进行平均。