Question

我创建了一个SELECT语句，用于从现在到我数据库中的某个日期获取时差。

SELECT
    `username`,
    TIMESTAMPDIFF(HOUR, `a_date`, NOW()) AS `timediff`
FROM
    `User`

...会产生类似

的内容

username | timediff
-------------------
john     |      441
henry    |     1624
mike     |        4
kyle     |     NULL

现在我想将它们分组到预先定义的组中，例如

NULL
less than 20
greater than 20

如何使用一组静态预定义值JOIN上述结果，以及如何将这些值用作我的分组标识符？

group | amount
--------------
NULL  |      1
lt 20 |      1
gt 20 |      2

Answer 1

您可以将case语句与group by：

一起使用

SELECT (CASE WHEN TIMESTAMPDIFF(HOUR, `a_date`, NOW()) < 20 THEN 'lt 20'
             WHEN TIMESTAMPDIFF(HOUR, `a_date`, NOW()) >= 20 THEN 'gt 20'
        END) as grp, COUNT(*)
FROM `User`
GROUP BY grp;

作为旁注：我通常会在case中重复group by语句，因为许多数据库不允许使用列别名：

SELECT (CASE WHEN TIMESTAMPDIFF(HOUR, `a_date`, NOW()) < 20 THEN 'lt 20'
             WHEN TIMESTAMPDIFF(HOUR, `a_date`, NOW()) >= 20 THEN 'gt 20'
        END) as grp, COUNT(*)
FROM `User`
GROUP BY (CASE WHEN TIMESTAMPDIFF(HOUR, `a_date`, NOW()) < 20 THEN 'lt 20'
               WHEN TIMESTAMPDIFF(HOUR, `a_date`, NOW()) >= 20 THEN 'gt 20'
          END);

如何加入静态值并将它们用作GROUP BY标识符？

1 个答案: