Question

我有一张这样的表：

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

。。

我希望在一个查询中获得多雨的星期一和阳光明媚的星期一，如

day     rainy_d  sunny_d
Monday  2        5

如何在Mysql和PostgreSQL中实现它？

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

标准SQL，适用于：

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

更简洁的版本 - 仅限MySQL：

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

更简洁的版本 - 仅限PostgreSQL：

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

Answer 3

列day可能是多余的。我会删除它而不用替换。列date包含所有信息。然后你的查询看起来像这样......

在PostgreSQL中：

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中：

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

NULLIF()构造适用于weather列中的两个不同（非空）值，并且是标准SQL。有关更多值，请使用@Mark和@xQbert提供的替代方法。

行计数，其中column1 ='value1'，column1 ='value2'group by column2

3 个答案: