行計數column1在= 'VALUE1' 和列1 = '值' 組由COLUMN2

Question

我有這樣一個表：

date   day  weather 
2000-01-01 Monday Sunny 
2000-01-02 Tuesday Rainy 

。 。 。

我想多雨星期一和陽光明媚的週一的數量在一個查詢像

day  rainy_d sunny_d 
Monday 2  5 

如何做到在MySQL和PostgreSQL？

Answer 1

select `Day`, 
SUM(case when weather = 'Sunny' THEN 1 ELSE 0 end) as Sunny_D, 
SUM(case when weather = 'Rainy' THEN 1 ELSE 0 end) as Rainy_D 
FROM YOURTABLENAME 
Where day = 'Monday' 
Group by `Day` 

Answer 2

標準SQL，工作在兩個：

SELECT 
    day, 
    SUM(CASE WHEN weather = 'Rainy' THEN 1 ELSE 0 END) AS rainy_d, 
    SUM(CASE WHEN weather = 'Sunny' THEN 1 ELSE 0 END) AS sunny_d 
FROM yourtable 
GROUP BY day 

更簡潔版本 - 只有MySQL的：

SELECT 
    day, 
    SUM(weather = 'Rainy') AS rainy_d, 
    SUM(weather = 'Sunny') AS sunny_d 
FROM yourtable 
GROUP BY day 

更簡潔版本 - 只有PostgreSQL的：

SELECT 
    day, 
    SUM((weather = 'Rainy')::int) AS rainy_d, 
    SUM((weather = 'Sunny')::int) AS sunny_d 
FROM yourtable 
GROUP BY day 

Answer 3

的列day可能是多餘的。我會刪除它而不用替換。列date保存所有信息。然後將查詢看起來是這樣的..

在PostgreSQL：

SELECT to_char(date, 'Day') AS day 
     ,COUNT(NULLIF(weather,'Sunny')) AS rainy_d 
     ,COUNT(NULLIF(weather,'Rainy')) AS sunny_d 
FROM tbl 
GROUP BY 1; 

在MySQL：

SELECT DAYNAME(date) AS day 
... rest identical 

的NULLIF()構造列工作了整整兩個不同的（非空）值weather並且是標準的SQL。有關更多值，請使用@Mark和@xQbert提供的替代方法。