mysql，groupby

在MySQL中，使用GROUP BY关键字可以对数据进行分组，并进行统计分组的信息。下面是一个示例：

SELECT id%10 AS m, COUNT(*) AS c
FROM t
GROUP BY m;

上述语句将根据id%10的值对数据进行分组，并统计每个分组中的记录数量。其中，id%10表示对id字段取模运算，得到的结果作为分组依据。

请注意，GROUP BY默认会对分组字段进行排序。如果没有排序要求，可以在SQL语句的最后加上`ORDER BY NULL`来取消排序，如下所示：

SELECT id%10 AS m, COUNT(*) AS c
FROM t
GROUP BY m
ORDER BY NULL;

这样可以避免不必要的排序操作，提高查询性能。

相关问题

mysql group by avg_mysql~group by

`GROUP BY` is a clause in MySQL that is used to group rows that have the same values in a specific column or columns.

`AVG` is a function in MySQL that is used to calculate the average of a set of values.

So, if you want to group rows by a specific column and then calculate the average for each group, you can use both `GROUP BY` and `AVG` together.

For example, let's say you have a table called "sales" with columns "region" and "sales_amount". You can use the following query to group the sales by region and calculate the average sales amount for each region:

SELECT region, AVG(sales_amount) FROM sales GROUP BY region;

This will return a result set that shows each region and its average sales amount.

mysql group by distinct

distinct和group by在MySQL中用于进行数据分组的操作。在MySQL 8.0之前，group by会进行隐式排序，可能导致性能下降，而distinct则没有这个问题。但是从MySQL 8.0开始，MySQL删除了隐式排序，所以在语义相同且无索引的情况下，group by和distinct的执行效率几乎是相同的。此外，group by和distinct都可以使用索引，它们的效率也是相同的，因为distinct可以被看作是特殊的group by。