Mysql5.7出现group by查询问题的解决办法分享
发现问题
最近在将mysql升级到mysql 5.7后，进行一些group by 查询时，比如下面的
SELECT *, count(id) as count FROM `news` GROUP BY `group_id` ORDER BY `inputtime` DESC LIMIT 20
就会报如下错误：
SELECT list is not in GROUP BY clause and contains nonaggregated column ‘news.id' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by.
原因分析
原因是mysql 5.7 模式中。默认启用了ONLY_FULL_GROUP_BY。
ONLY_FULL_GROUP_BY是MySQL提供的一个sql_mode，通过这个sql_mode来提供SQL语句GROUP BY合法性的检查。
http://dev.mysql.com/doc/refman/5.7/en/sql-mode.html#sqlmode_only_full_group_by
this is incompatible with sql_mode=only_full_group_by这句话提示了这违背了mysql的规则，only fully group by，也就是说在执行的时候先分组，根据查询的字段（select的字段）在分组的内容中取出，所以查询的字段全部都应该在group by分组条件内；一种情况例外，查询字段中如果含有聚合函数的字段不用包含在group by中，就像我上面的count（id）。
后来发现Order by排序条件的字段也必须要在group by内，排序的字段也是从分组的字段中取出。 不明白的可以去看一下。
解决办法：
1.set@@sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_pISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';
去掉ONLY_FULL_GROUP_BY即可正常执行sql.
2. 不去ONLY_FULL_GROUP_BY, 时 select字段必须都在group by分组条件内（含有函数的字段除外）。（如果遇到order by也出现这个问题，同理，order by字段也都要在group by内）。
3.利用ANY_VALUE()这个函数 https://dev.mysql.com/doc/refman/5.7/en/miscellaneous-functions.html#function_any-value
This function is useful for GROUP BY queries when the ONLY_FULL_GROUP_BY SQL mode is enabled, for cases when MySQL rejects a query that you know is valid for reasons that MySQL cannot determine. The function return value and type are the same as the return value and type of its argument, but the function result is not checked for the ONLY_FULL_GROUP_BY SQL mode.
如上面的sql语句可写成
SELECT ANY_VALUE(id)as id,ANY_VALUE(uid) as uid ,ANY_VALUE(username) as username,ANY_VALUE(title) as title,ANY_VALUE(author) as author,ANY_VALUE(thumb) as thumb,ANY_VALUE(description) as description,ANY_VALUE(content) as content,ANY_VALUE(linkurl) as linkurl,ANY_VALUE(url) as url,ANY_VALUE(group_id) as group_id,ANY_VALUE(inputtime) as inputtime, count(id) as count FROM `news` GROUP BY `group_id` ORDER BY ANY_VALUE(inputtime） DESC LIMIT 20
我选用的是第３种方法。
总结
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