Get variable MIN Date based on marker in row

问题

I'm using MySQL with phpMyAdmin, below is my data table:

+--------+---------+-----------------+
| userID | context |      time       |
+--------+---------+-----------------+
|    111 |         | 7/1/2021        |
|    111 |         | 7/16/2019       |
|    111 | Reset   | 7/15/2019       |
|    222 |         | 7/9/2020        |
|    222 | Reset   | 7/8/2020        |
|    333 | Reset   | 5/11/2020       |
|    333 |         | 5/10/2020       |
|    444 |         | 9/8/2020        |
+--------+---------+-----------------+

I'm looking for a SELECT query that gives me the MIN time greater or equal to the date where a Reset is logged in the context column. If no Reset marker exists in the context column, I'd like to have the result included.

So for the table above, I expect the result:

+--------+-----------------+
| userID |      time       |
+--------+-----------------+
|    111 | 7/15/2019       |
|    222 | 7/8/2020        |
|    333 | 5/11/2020       |
|    444 | 9/8/2020        |
+--------+-----------------+

I tried :

SELECT * FROM foo as a 
WHERE ((SELECT MIN(a.time) FROM foo) >= (SELECT Max(a.time) FROM foo where context = 'Reset')) group by userID

does not produce my expected output, but returns:

+--------+-----------+
| userID |   time    |
+--------+-----------+
|    111 | 7/1/2021  |  <--- wrong 
|    222 | 7/8/2020  |
|    333 | 5/11/2020 |
+--------+-----------+

回答1:

Basis on your expected result, You may try below query -

SELECT userID, MAX(time)
  FROM YOUR_TABLE
 WHERE context = 'RESET'
 GROUP BY userID
 UNION ALL
SELECT userID, MAX(time)
  FROM YOUR_TABLE
 WHERE context IS NULL
 GROUP BY userID
 ORDER BY userID

回答2:

Use window functions:

select t.*,
       coalesce(next_time, time) as imputed_time
from (select t.*,
             sum(context = 'reset') over (partition by user_id) as cnt_reset,
             min(case when context is null then time end) over (partition by userid order by time rows between current row and unbounded following) as next_time
      from t
     ) t
where cnt_reset = 0 or context = 'reset';

来源：https://stackoverflow.com/questions/62817481/get-variable-min-date-based-on-marker-in-row

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