MySQL和多列的平均值+每列的单独条件

Question

我有以下查询：

SELECT `station_id`, AVG(`jan`) AS avg_jan, AVG(`feb`) AS avg_feb, ... , 
AVG(`dec`) AS avg_dec 
FROM `climate_data`

WHERE `element_name` = "Temp_mean_mly" 
AND `jan` <> -999999 
AND `feb` <> -999999
AND ... 
AND `dec` <> -999999 
GROUP BY station_id 

我需要它为每个列单独返回不同于-999999的平均值！当前查询将删除所有具有值-999999且不正确的列的行。

由于

Answer 1

你可以这样使用NULLIF：

SELECT 
    station_id, 
    AVG(NULLIF(jan, -99999)) AS avg_jan, 
    AVG(NULLIF(feb, -99999)) AS avg_feb,
    ...
FROM climate_data
WHERE element_name = 'Temp_mean_mly'
GROUP BY station_id 

这是有效的，因为聚合函数会跳过NULL值。