Question

我发现很难绕过这个。例如，如果我有一个像这样的表：

table: cat
  -------------------------------------------------------------
  | id       | currency   |bank         |created_at           |
  -------------------------------------------------------------
  | 1        | a          |1            |2015-05-12 19:30:55  |
  | 2        | a          |1            |2015-05-12  20:30:55 |
  | 3        | b          |2            |2015-05-12  19:30:55 |
  | 4        | s          |1            |2015-05-13 19:30:55  |
  | 5        | b          |2            |2015-05-13  23:30:55 |
  | 6        | b          |2            |2015-05-13  19:30:55 |
  ----------------------------------------------------

我的预期结果：

 -------------------------------------------------------------
  | id       | currency   |bank         |created_at           |
  -------------------------------------------------------------
  | 2        | a          |1            |2015-05-12  20:30:55 |
  | 3        | b          |2            |2015-05-12  19:30:55 |
  | 4        | s          |1            |2015-05-13 19:30:55  |
  | 5        | b          |2            |2015-05-13  23:30:55 |
  ----------------------------------------------------

我的尝试：

SELECT p.id,p.currencyname, p.bank, date(p.created_at) as d, max(time(created_at)) as t, p.currencyname from cat as p  GROUP by p.currencyname, d

Answer 1

看起来您想要每个日历日的每种货币的最新（最大时间值）行。正如您专业地使用SQL一样，尊重您，花时间思考您的要求并准确描述它们至关重要。

以下是您获得所需结果的方法。

首先，您希望在每个日历日（created_at）中找到每种货币的最新（MAX()）行的DATE()值。执行此操作的查询如下所示（http://sqlfiddle.com/#!9/a0e097/1/0）：

            SELECT DATE(created_at) day, currency, MAX(created_at) created_at
              FROM cat
             GROUP BY DATE(created_at), currency

然后将其用作子查询，并将其加入到您的表中。连接将从表中提取详细记录。你订购它是有道理的（http://sqlfiddle.com/#!9/a0e097/2/0）。

 SELECT cat.id, cat.currency, cat.bank, cat.created_at
   FROM cat
   JOIN (
                 SELECT DATE(created_at) day, currency, MAX(created_at) created_at
                  FROM cat
                 GROUP BY DATE(created_at), currency
        ) a ON cat.currency = a.currency AND cat.created_at = a.created_at
  ORDER BY cat.id

如何在mysql表中选择最新值行2天

1 个答案: