How do I count multiple columns of events for users?

问题

I have a table of events that stores the events for users and I want to get the number of two specific events for each user.

An example table is called "events" and it has 2 columns

user_id VARCHAR(50)
event_name VARCHAR(50)

The user_ids are all unique and the event names can be things like login, sent_message, liked_post

How do I, for example, query for the total messages sent per user AND the number of liked_posts per user?

回答1:

This is the pattern of query you can use:

select user_id, 
    sent_message_count=sum(case when event_name = 'Sent Message' then 1 else 0 end), 
    liked_post_count=sum(case when event_name = 'Liked Post' then 1 else 0 end),
from events
group by user_id

Now, you just need to make sure the when part of each case statement fits the criteria you need. The pattern itself - summing a bunch of 1's where the criteria fits, is really the key to achieving the result you're after.

回答2:

SELECT user_id, 
    SUM(IF(event_name = 'sent_message', 1, 0)) AS sent_message,  
    SUM(IF(event_name = 'liked_posts', 1, 0)) AS liked_posts
FROM `events`
GROUP BY user_id

来源：https://stackoverflow.com/questions/35944845/how-do-i-count-multiple-columns-of-events-for-users