發現問題

最近在將mysql升級到mysql 5.7后，進行一些group by 查詢時，比如下面的

SELECT *, count(id) as count FROM `news` GROUP BY `group_id` ORDER BY `inputtime` DESC LIMIT 20

就會報如下錯誤：

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。默認啟用了ONLY_FULL_GROUP_BY。

ONLY_FULL_GROUP_BY是MySQL提供的一個sql_mode，通過這個sql_mode來提供SQL語句GROUP BY合法性的檢查。

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這句話提示了這違背了mysql的規則，only fully group by，也就是說在執行的時候先分組，根據查詢的字段（select的字段）在分組的內容中取出，所以查詢的字段全部都應該在group by分組條件內；一種情況例外，查詢字段中如果含有聚合函數的字段不用包含在group by中，就像我上面的count（id）。

后來發現Order by排序條件的字段也必須要在group by內，排序的字段也是從分組的字段中取出。 不明白的可以去看一下。

解決辦法：

1.set@@sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';

去掉ONLY_FULL_GROUP_BY即可正常執行sql.

2. 不去ONLY_FULL_GROUP_BY, 時 select字段必須都在group by分組條件內（含有函數的字段除外）。（如果遇到order by也出現這個問題，同理，order by字段也都要在group by內）。

3.利用ANY_VALUE()這個函數　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語句可寫成

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

我選用的是第３種方法。

總結

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