Android SQLite按频率排序条目而不进行分组

Question

I am trying to sort the database records by row value frequency in descending order but without grouping the entries. Excuse me, I am a total beginner in databases.

Here is the full code:

SELECT   * 
FROM     table_name 
WHERE    date_time >= 10 
and      date_time <= 100 
GROUP BY activity_type 
ORDER BY COUNT(activity_type) DESC

And here is the relevant code:

SELECT   * 
FROM     table_name 
GROUP BY activity_type 
ORDER BY COUNT(activity_type) DESC

This is the desired result (sorted by the frequency of activity_type in a descending order without grouping):

name (irrelevant)    activity_type (number)
------------------------------------------------
Rome                       1
London                     1
Madrid                     1
Stockholm                  3
Paris                      3
Moscow                     2

Answer 1

Unfortunately, SQLite doesn't support group by in subqueries. So your choices are either an outer join (which creates a cross product of the table by itself and which is then culled with a group by ) or a temporary table which would contain activity_type counts.

CREATE TABLE activity_count_table(activity_type, activity_count);

INSERT INTO activity_count_table 
    SELECT activity_type, COUNT(activity_type) AS activity_count
    FROM table_name
    WHERE date_time BETWEEN 10 AND 100
    GROUP BY activity_type;

SELECT * 
FROM table_name NATURAL JOIN activity_count_table
WHERE date_time BETWEEN 10 AND 100
ORDER BY activity_count_table.activity_count DESC;

DROP TABLE activity_count_table;

Obviously, this is not an ideal solution. You probably want to have some unique identifier added to the name activity_count_table for each call (so that multiple queries don't interfere with each other).

It's still better than creating an outer join (which needs n^2 rows in memory) and then culling it with group by (which really just gives you a diagonal when applied to a self-join).

Answer 2

If the aggregation must not affect the actual query, you have to move it into a subquery:

SELECT *
FROM table_name
WHERE date_time BETWEEN 10 AND 100
ORDER BY (SELECT count(*)
          FROM table_name AS T2
          WHERE T2.activity_type = table_name.activity_type
            AND date_time BETWEEN 10 AND 100) DESC;

Answer 3

If your database supports it, you can use a windowing function with the OVER clause, eg

SELECT *
  FROM table_name 
 ORDER BY COUNT(*) OVER (PARTITION BY activity_type) DESC

Confirmed to work on:

• MS SQL Server 2008 R2
• PostgreSQL 9.3