ID | asker 1 | Bob 2 | Bob 3 | Marley
我想选择每个asker只有一次,如果有多个同名的问题,选择最高的id之一.所以,预期的结果：
ID | asker 3 | Marley 2 | Bob
我使用以下查询：
SELECT * FROM questions GROUP by questions.asker ORDER by questions.id DESC
我得到以下结果：
ID | asker 3 | Marley 1 | Bob
所以它选择了遇到的第一个“Bob”,而不是最后一个.
谢谢
SELECT max(id) as id,asker FROM questions GROUP by asker ORDER by id DESC
您获得异常结果的原因是因为MysqL使用GROUP BY的扩展,它允许选择列表中的项目不分组,而不包括在GROUP BY子句中.这可能会导致意想不到的结果,因为MysqL可以选择返回的值. (见MySQL Extensions to GROUP BY)
从MysqL文档：
MysqL extends the use of GROUP BY so that the select list can refer to nonaggregated columns not named in the GROUP BY clause. … You can use this feature to get better performance by avoiding unnecessary column sorting and grouping. However,this is useful primarily when all values in each nonaggregated column not named in the GROUP BY are the same for each group. The server is free to choose any value from each group,so unless they are the same,the values chosen are indeterminate. Furthermore,the selection of values from each group cannot be influenced by adding an ORDER BY clause. Sorting of the result set occurs after values have been chosen,and ORDER BY does not affect which values the server chooses.
