类似于MySQL枢轴的操作，以获取每种事件类型每天总事件数的细分 [英] MySQL pivot-like operation to get breakdown of percentage of total events per day per event type

问题描述

有一张事件表

created_at DATETIME
event_type STRING 
# Some other columns with data about the event

我想做的是能够获取每个event_type每天事件总数的百分比.

What I would like to do is be able to get the percentage of the total number of events per day per event_type.

因此，我将事件分组以获取每个事件每天的计数:

So, I grouped the events to get the count per day per event:

# Lazily used date_bucket in GROUP BY since it's valid MySQL.
# Is that bad since it's not standard SQL?
#
SELECT 
    DATE(created_at) as date_bucket, 
    event_type, 
    COUNT(*) as number
FROM 
    example_table 
GROUP BY
    date_bucket, event_type

如果我们有行

# Columns (date_bucket, event_type, number)
#
('2020-06-02', 'exampleG1', 5)
('2020-06-02', 'exampleG2', 10)
('2020-06-02', 'exampleG3', 20)
('2020-06-03', 'exampleG1', 10)

我希望能够获得与

# Columns (date_bucket, exampleG1, exampleG2, exampleG3)
#
('2020-06-02', 15/35, 10/35, 20/35)
('2020-06-03', 10/10, 0, 0)

我之前不知道不同的event_type值，并且并非所有的组值可能全天都存在，在这种情况下，该类型的值应在当天为0.

I don't know the distinct event_type values before hand, and not all group values may be present on all days, in which case the value for that type should be 0 on that day.

我当时正在考虑进行某种枢轴操作，但是MySQL似乎不支持枢轴，因此我对如何实现这种方法有点茫然.

I was thinking to do some kind of pivot operation, but it appears MySQL doesn't support pivots, so I'm at a bit of a loss how to approach this.

如果我提前知道有效事件类型的集合，我想我可以对可能的类型进行一些讨厌的冗长查询，但是该集合是可变的.

If I knew the set of valid event types ahead of time, I think I could do some nasty verbose query on the possible types, but the set is variable.

是否有一种优雅的方法来实现这一目标?

Is there an elegant way of achieving this?

推荐答案

我不知道事前有哪些不同的event_type值

I don't know the distinct event_type values before hand

您正在请求动态SQL.即，从另一个列出了不同的event_type值的查询中动态构建查询字符串，然后执行它.在MySQL中，这是使用准备好的语句实现的.

You are asking for dynamic SQL. That is, dynamically build the query string from another query that lists distinct event_type values, then execute it. In MySQL, this is implemented using prepared statements.

这是操作方法:

select @sql := group_concat(distinct
    'sum(case when event_type = ''', 
    event_type, ''' then number else 0 end)/sum(number) as `ratio_', 
    event_type, '`'
) 
from example_table;

set @sql = concat(
    'select date(created_at) date_bucket, ', 
    @sql, 
    ' from example_table group by date(created_at) order by date_bucket'
);

-- debug
select @sql;

-- execute
prepare stmt from @sql;
execute stmt;
deallocate prepare stmt; 

对于您的示例数据，这将产生以下查询:

For your sample data, this produces the following query:

select 
    date(created_at) date_bucket, 
    sum(case when event_type = 'exampleG1' then number else 0 end)/sum(number) as `ratio_exampleG1`,
    sum(case when event_type = 'exampleG2' then number else 0 end)/sum(number) as `ratio_exampleG2`,
    sum(case when event_type = 'exampleG3' then number else 0 end)/sum(number) as `ratio_exampleG3` 
from example_table 
group by date(created_at) 
order by date_bucket

以及以下结果:

date_bucket | ratio_exampleG1 | ratio_exampleG2 | ratio_exampleG3
:---------- | --------------: | --------------: | --------------:
2020-06-02  |          0.1429 |          0.2857 |          0.5714
2020-06-03  |          1.0000 |          0.0000 |          0.0000

DB Fiddle上的演示

Demo on DB Fiddle

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