已加入并计数的mysql查询

Question

I need help getting the top 5 results and their counts from columns from two different tables in a mysql database joined together.

table1 cols
-------
id, country, timestamp

table2 cols
--------
id, table1_id, reason

The results id like to get are the top 5 countries and their number of times found between two timestamps, and the top 5 reasons and their counts for all the rows used to generate the first count. There is a one to many relationship between table1 and table2. This is stumping me and I appreciate any insight you could give me.

Answer 1

It's not entirely clear what resultset you want to return.

This may be of some help to you:

SELECT t.country
     , COUNT(DISTINCT t.id) AS count_table1_rows
     , COUNT(r.id)          AS count_table2_rows
     , COUNT(*)             AS count_total_rows
  FROM table1 t
  LEFT
  JOIN table2 r
    ON r.table1_id = t.id
 WHERE t.timestamp >= NOW() - INTERVAL 7 DAY
   AND t.timestamp  < NOW()
 GROUP BY t.country
 ORDER BY COUNT(DISTINCT t.id) DESC
 LIMIT 5

That will return a maximum of 5 rows, one row per country, with counts of rows in table1, counts of rows found in table2, and a count of the total rows returned.

The LEFT keyword specifies an "outer" join operation, such that rows from table1 are returned even if there are no matching rows found in table2.

To get the count for each "reason", associated with each country, you could do something like this:

SELECT t.country
     , COUNT(DISTINCT t.id) AS count_table1_rows
  FROM table1 t
  LEFT
  JOIN ( SELECT s.country
              , r.reason
              , COUNT(*) AS cnt_r
           FROM table1 s 
           JOIN table2 r
             ON s.table1_id = t.id
          WHERE s.timestamp >= NOW() - INTERVAL 7 DAY
            AND s.timestamp  < NOW()
          GROUP
             BY s.country
              , r.reason
       ) u
    ON u.country = t.country 
 WHERE t.timestamp >= NOW() - INTERVAL 7 DAY
   AND t.timestamp  < NOW()
 GROUP
    BY t.country
     , u.reason
 ORDER
    BY COUNT(DISTINCT t.id) DESC
     , t.country DESC
     , u.cnt_r DESC
     , u.reason DESC

This query doesn't "limit" the rows being returned. It would be possible to modify the query to have only a subset of the rows returned, but that can get complex. And before we muck the complexity of adding "top 5 within top 5" type limits, we want to ensure that the rows returned by a query are a superset of the rows we actually want.

Answer 2

Is this what you want?

select t2.reason, count(*)
from (select t1.country, count(*)
      from table1 t1
      where timestamp between @STARTTIME and @ENDTIME
      group by country
      order by count(*) desc
      limit 5
     ) c5 join
     table1 t1
     on c5.country = t1.country and
        t1.timestamp between @STARTTIME and @ENDTIME join
     table2 t2
     on t2.table1_id = t1.id
group by t2.reason;

The c5 subquery gets the five countries. The other two bring back the data for the final aggregation.