当我选择MAX(updated_date)时，其他列的值错误 [英] Getting wrong values for other columns when I select MAX(updated_date)

问题描述

我正在使用select field1, max(updated_date) from mytable.
我得到了max(updated_date)的正确值，即最大日期.
但是，对于field1，当我真的想要第三条记录(具有最大日期值的记录)中的"ta3"值时，我只获得第一条记录的值，即"ta1".

I am using select field1, max(updated_date) from mytable.
I get the correct value for max(updated_date), i.e. the largest date.
However for field1 I just get the the value for the first record, i.e. "ta1" when I really want the "ta3" value from the third record (the one with the max date value).

例如

+------------+---------------------+
| field1     | update_date         |
+------------+---------------------+
| ta1        | 2012-03-11 11:05:15 |
| ta2        | 2012-03-11 11:05:32 |
| ta3        | 2012-03-11 11:05:56 |
+------------+---------------------+
3 rows in set (0.00 sec)

+------------+---------------------+
| field1     | max(update_date)    |
+------------+---------------------+
| ta1        | 2012-03-11 11:05:56 |
+------------+---------------------+
1 row in set (0.00 sec)

推荐答案

您需要GROUP BY子句或更复杂的查询.

You either need a GROUP BY clause or a more complex query.

SELECT field1, MAX(updated_date)
  FROM mytable
 GROUP BY field1

对于示例数据，这将返回3行.

For the sample data, this will return 3 rows.

您更有可能想要:

SELECT t1.field1, t3.max_date
  FROM mytable AS t1
  JOIN (SELECT MAX(t2.updated_date) AS max_date
          FROM mytable AS t2
       ) AS t3
    ON t1.updated_date = t3.max_date;

对于示例数据，这将返回1行:

For the sample data, this will return 1 row:

ta3   2012-03-11 11:05:56

主要DBMS中，只有MySQL允许省略GROUP BY子句当你在选择列表中的聚集体和非聚合列的混合物. SQL标准需要GROUP BY子句，并且您必须在其中列出所有非聚合列.有时，在MySQL中，省略GROUP BY子句会产生所需的答案；不过，它常常能够给出意外的答案.

Of the major DBMS, only MySQL allows you to omit the GROUP BY clause when you have a mixture of aggregates and non-aggregate columns in the select-list. The SQL standard requires the GROUP BY clause and you must list all non-aggregate columns in it. Sometimes, in MySQL, omitting the GROUP BY clause produces the answer you want; as often as not, though, it manages to give an unexpected answer.

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