MySQL count on precondition?

问题

Consider the scheme below

id   user   tag    created
1    foo    tag1   2018-09-01
2    foo    tag2   2018-09-01
3    bar    tag1   2018-09-02
4    bar    tag2   2018-09-03
5    kkk    tag2   2018-09-05
6    qqq    tag1   2018-09-12

How to find the number of unique user, which after the row 'tag1' there is a 'tag2' followed in future row of that user?

The answer is 2 (foo,bar), based on the above query

回答1:

You can use aggregation to get the list of users:

select user
from t
group by user
having min(case when tag = 'tag1' then created end) < max(case when tag = 'tag2' then created end);

To get the number of such users, use a subquery:

select count(*)
from (select user
      from t
      group by user
      having min(case when tag = 'tag1' then created end) < max(case when tag = 'tag2' then created end)
     ) u;

回答2:

You can use a sub query to find such rows:

SELECT *
FROM yourdata AS t
WHERE tag = 'tag1'
AND EXISTS (
    SELECT 1
    FROM yourdata AS x
    WHERE x.user = t.user
    AND x.tag = 'tag2'
    AND (x.created > t.created OR x.id > t.id)
)

来源：https://stackoverflow.com/questions/52366225/mysql-count-on-precondition