mysql - column1='value1' and column1='value2' 按 column2 分组的行数

Question

我有一张这样的桌子：

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

. . .

我想在一个查询中获得下雨的星期一和晴天的星期一的数量，例如

day     rainy_d  sunny_d
Monday  2        5

如何在 Mysql 和 PostgreSQL 中实现这一点？

Answer 1

标准 SQL，适用于以下两种情况：

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 2

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 3

该列day可能是多余的。我会删除它而不替换。该列date包含所有信息。那么您的查询可能看起来像这样..

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

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