匹配计算

Question

选择行我有这个疑问来计算一周中最常见的日期和时间：

SELECT 
    DATE_FORMAT(DATE, '%a') AS wd, 
    HOUR(DATE) AS hr, 
    COUNT(*) AS cnt 
FROM 
users 
GROUP BY 
    wd, hr 
ORDER BY 
    cnt DESC 

现在，我要选择符合wd = "Fri" AND hr = "9"

我的所有行的ID的已经尝试添加WHERE子句，但由于我不能在别名上使用WHERE，所以我无法做到这一点。

Answer 1

SELECT ID 
FROM users 
WHERE DATE_FORMAT(DATE, '%a') = 'Fri' AND HOUR(DATE) = 9 

Answer 2

SELECT 
    DATE_FORMAT(DATE, '%a') AS wd, 
    HOUR(DATE) AS hr, 
    COUNT(*) AS cnt 
FROM users 
WHERE DATE_FORMAT(DATE, '%a') = 'Fri' 
AND HOUR(DATE) = 9 
GROUP BY wd, hr 
ORDER BY cnt DESC 

你可以这样做，也是使用别名的选择，但性能会不一样好，因为这样一来过滤正在发生的表的份额前聚集