SQL: selecting top N records per group

January 6, 2011

A while back I presented(*) an SQL trick to present with non-aggregated column on a GROUP BY query, without use of subquery or derived tables.

Based on a similar concept, combined with string walking, I now present a query which selects top-n records for each group, ordered by some condition. It will require no subqueries. It executes faster than its more conventional alternatives.

[UPDATE: this is MySQL only. Others can use Window Functions where available]

Using the simple world database, we answer the following question:

What are the top 5 largest (by area) countries for each continent? What are their names, surface area and population?

Similar questions would be:

What were the latest 5 films rented by each customer?

What were the most presented advertisements for each user?

etc.

Step 1: getting the top

We already know how to get a single column's value for the top country, as presented in the aforementioned post:

SELECT
 Continent,
 SUBSTRING_INDEX(
   GROUP_CONCAT(Name ORDER BY SurfaceArea DESC),
   ',', 1) AS Name
FROM
 Country
GROUP BY
 Continent
;
+---------------+--------------------+
| Continent     | Name               |
+---------------+--------------------+
| Asia          | China              |
| Europe        | Russian Federation |
| North America | Canada             |
| Africa        | Sudan              |
| Oceania       | Australia          |
| Antarctica    | Antarctica         |
| South America | Brazil             |
+---------------+--------------------+

Step 2: adding columns

This part is easy: just throw in the rest of the columns (again, only indicating the top country in each continent)

SELECT
 Continent,
 SUBSTRING_INDEX(
   GROUP_CONCAT(Name ORDER BY SurfaceArea DESC),
   ',', 1) AS Name,
 SUBSTRING_INDEX(
   GROUP_CONCAT(SurfaceArea ORDER BY SurfaceArea DESC),
   ',', 1) AS SurfaceArea,
 SUBSTRING_INDEX(
   GROUP_CONCAT(Population ORDER BY SurfaceArea DESC),
   ',', 1) AS Population
FROM
 Country
GROUP BY
 Continent
;

+---------------+--------------------+-------------+------------+
| Continent     | Name               | SurfaceArea | Population |
+---------------+--------------------+-------------+------------+
| Asia          | China              | 9572900.00  | 1277558000 |
| Europe        | Russian Federation | 17075400.00 | 146934000  |
| North America | Canada             | 9970610.00  | 31147000   |
| Africa        | Sudan              | 2505813.00  | 29490000   |
| Oceania       | Australia          | 7741220.00  | 18886000   |
| Antarctica    | Antarctica         | 13120000.00 | 0          |
| South America | Brazil             | 8547403.00  | 170115000  |
+---------------+--------------------+-------------+------------+

Step 3: casting

You'll notice that the Population column from this last execution is aligned to the left. This is because it is believed to be a string. The GROUP_CONCAT clause concatenates values in one string, and SUBSTRING_INDEX parses a substring. The same applies to the SurfaceArea column. We'll cast Population as UNSIGNED and SurfaceArea as DECIMAL:

SELECT
  Continent,
  SUBSTRING_INDEX(
    GROUP_CONCAT(Name ORDER BY SurfaceArea DESC),
    ',', 1) AS Name,
  CAST(
    SUBSTRING_INDEX(
      GROUP_CONCAT(SurfaceArea ORDER BY SurfaceArea DESC),
      ',', 1)
    AS DECIMAL(20,2)
    ) AS SurfaceArea,
  CAST(
    SUBSTRING_INDEX(
      GROUP_CONCAT(Population ORDER BY SurfaceArea DESC),
      ',', 1)
    AS UNSIGNED
    ) AS Population
FROM
 Country
GROUP BY
 Continent
;
+---------------+--------------------+-------------+------------+
| Continent     | Name               | SurfaceArea | Population |
+---------------+--------------------+-------------+------------+
| Asia          | China              |  9572900.00 | 1277558000 |
| Europe        | Russian Federation | 17075400.00 |  146934000 |
| North America | Canada             |  9970610.00 |   31147000 |
| Africa        | Sudan              |  2505813.00 |   29490000 |
| Oceania       | Australia          |  7741220.00 |   18886000 |
| Antarctica    | Antarctica         | 13120000.00 |          0 |
| South America | Brazil             |  8547403.00 |  170115000 |
+---------------+--------------------+-------------+------------+

Step 4: top n records

It's time to use string walking. Examples for string walking (described in the excellent SQL Cookbook) can be found here, here and here. We'll be using a numbers table: a simple table which lists ascending integer numbers. For example, you can use the following:

DROP TABLE IF EXISTS `tinyint_asc`;

CREATE TABLE `tinyint_asc` (
 `value` tinyint(3) unsigned NOT NULL default '0',
 PRIMARY KEY (value)
) ;

INSERT INTO `tinyint_asc` VALUES (0),(1),(2),(3),(4),(5),(6),(7),(8),(9),(10),(11),(12),(13),(14),(15),(16),(17),(18),(19),(20),(21),(22),(23),(24),(25),(26),(27),(28),(29),(30),(31),(32),(33),(34),(35),(36),(37),(38),(39),(40),(41),(42),(43),(44),(45),(46),(47),(48),(49),(50),(51),(52),(53),(54),(55),(56),(57),(58),(59),(60),(61),(62),(63),(64),(65),(66),(67),(68),(69),(70),(71),(72),(73),(74),(75),(76),(77),(78),(79),(80),(81),(82),(83),(84),(85),(86),(87),(88),(89),(90),(91),(92),(93),(94),(95),(96),(97),(98),(99),(100),(101),(102),(103),(104),(105),(106),(107),(108),(109),(110),(111),(112),(113),(114),(115),(116),(117),(118),(119),(120),(121),(122),(123),(124),(125),(126),(127),(128),(129),(130),(131),(132),(133),(134),(135),(136),(137),(138),(139),(140),(141),(142),(143),(144),(145),(146),(147),(148),(149),(150),(151),(152),(153),(154),(155),(156),(157),(158),(159),(160),(161),(162),(163),(164),(165),(166),(167),(168),(169),(170),(171),(172),(173),(174),(175),(176),(177),(178),(179),(180),(181),(182),(183),(184),(185),(186),(187),(188),(189),(190),(191),(192),(193),(194),(195),(196),(197),(198),(199),(200),(201),(202),(203),(204),(205),(206),(207),(208),(209),(210),(211),(212),(213),(214),(215),(216),(217),(218),(219),(220),(221),(222),(223),(224),(225),(226),(227),(228),(229),(230),(231),(232),(233),(234),(235),(236),(237),(238),(239),(240),(241),(242),(243),(244),(245),(246),(247),(248),(249),(250),(251),(252),(253),(254),(255);

The trick is to apply the same technique as used above, not for a single row, but for several rows. Here's how to present the top 5 countries:

SELECT
  Continent,
  SUBSTRING_INDEX(
    SUBSTRING_INDEX(
      GROUP_CONCAT(Name ORDER BY SurfaceArea DESC),
      ',', value),
    ',', -1)
    AS Name,
  CAST(
    SUBSTRING_INDEX(
      SUBSTRING_INDEX(
        GROUP_CONCAT(SurfaceArea ORDER BY SurfaceArea DESC),
        ',', value),
      ',', -1)
    AS DECIMAL(20,2)
    ) AS SurfaceArea,
  CAST(
    SUBSTRING_INDEX(
      SUBSTRING_INDEX(
        GROUP_CONCAT(Population ORDER BY SurfaceArea DESC),
        ',', value),
      ',', -1)
    AS UNSIGNED
    ) AS Population
FROM
  Country, tinyint_asc
WHERE
  tinyint_asc.value >= 1 AND tinyint_asc.value <= 5
GROUP BY
  Continent, value
;
+---------------+----------------------------------------------+-------------+------------+
| Continent     | Name                                         | SurfaceArea | Population |
+---------------+----------------------------------------------+-------------+------------+
| Asia          | China                                        |  9572900.00 | 1277558000 |
| Asia          | India                                        |  3287263.00 | 1013662000 |
| Asia          | Kazakstan                                    |  2724900.00 |   16223000 |
| Asia          | Saudi Arabia                                 |  2149690.00 |   21607000 |
| Asia          | Indonesia                                    |  1904569.00 |  212107000 |
| Europe        | Russian Federation                           | 17075400.00 |  146934000 |
| Europe        | Ukraine                                      |   603700.00 |   50456000 |
| Europe        | France                                       |   551500.00 |   59225700 |
| Europe        | Spain                                        |   505992.00 |   39441700 |
| Europe        | Sweden                                       |   449964.00 |    8861400 |
| North America | Canada                                       |  9970610.00 |   31147000 |
| North America | United States                                |  9363520.00 |  278357000 |
| North America | Greenland                                    |  2166090.00 |      56000 |
| North America | Mexico                                       |  1958201.00 |   98881000 |
| North America | Nicaragua                                    |   130000.00 |    5074000 |
| Africa        | Sudan                                        |  2505813.00 |   29490000 |
| Africa        | Algeria                                      |  2381741.00 |   31471000 |
| Africa        | Congo                                        |  2344858.00 |   51654000 |
| Africa        |  The Democratic Republic of the              |  1759540.00 |    5605000 |
| Africa        | Libyan Arab Jamahiriya                       |  1284000.00 |    7651000 |
| Oceania       | Australia                                    |  7741220.00 |   18886000 |
| Oceania       | Papua New Guinea                             |   462840.00 |    4807000 |
| Oceania       | New Zealand                                  |   270534.00 |    3862000 |
| Oceania       | Solomon Islands                              |    28896.00 |     444000 |
| Oceania       | New Caledonia                                |    18575.00 |     214000 |
| Antarctica    | Antarctica                                   | 13120000.00 |          0 |
| Antarctica    | French Southern territories                  |     7780.00 |          0 |
| Antarctica    | South Georgia and the South Sandwich Islands |     3903.00 |          0 |
| Antarctica    | Heard Island and McDonald Islands            |      359.00 |          0 |
| Antarctica    | Bouvet Island                                |       59.00 |          0 |
| South America | Brazil                                       |  8547403.00 |  170115000 |
| South America | Argentina                                    |  2780400.00 |   37032000 |
| South America | Peru                                         |  1285216.00 |   25662000 |
| South America | Colombia                                     |  1138914.00 |   42321000 |
| South America | Bolivia                                      |  1098581.00 |    8329000 |
+---------------+----------------------------------------------+-------------+------------+

Limitations

You should have:

  • Enough numbers in the numbers table (I've used 5 out of 255)
  • Reasonable setting for group_concat_max_len (see this post). Actually it would be better to have a smaller value here, while you make sure it's large enough; this way you do not waste memory for large groups.

(*) This was two years ago! I'm getting old

Update: see also

Another hack at same problem: SQL: selecting top N records per group, another solution

tags:
posted in MySQL by shlomi

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27 Comments to "SQL: selecting top N records per group"

  1. Shantanu Oak wrote:

    There is no such country "The Democratic Republic of the" in Africa. It seems the full name of Congo is being displayed as second nation. There is another way of calculating top N records. Here...

    http://oksoft.blogspot.com/2011/01/top-n-records-per-group.html

  2. Pabloj wrote:

    Of course there's also a standard way of doing this (supported by Pg, Oracle, DB2 etc.) which is using window functions ...

  3. shlomi wrote:

    @Pabloj,
    Quite so... :)
    GROUP_CONCAT "solves" the missing features of window functions.

  4. shlomi wrote:

    @shantanu,
    Your solution involves a table creation followed by INSERTs.
    In fact, it involves copying the entire table. So this is a serious consideration IMHO.

  5. SQL wrote:

    That means the total cost is SLOW

  6. shlomi wrote:

    @SQL
    As I said, I happened to see it executes faster than the subquery alternatives.

  7. Jonathan Levin wrote:

    Love it. Brilliant.
    Really good post.

  8. huarong wrote:

    great post for group_concat and string walking

  9. huarong wrote:

    where is the world database ?

  10. shlomi wrote:

    @huarong,
    Seems like it has been moved. I've updated the link.

  11. dtecmeister wrote:

    create table CountryRankBySurfaceAreaPerContinent (
      Code char(3) NOT NULL DEFAULT '',
      Rank int(4) not null default '0',
      PRIMARY KEY (`Code`)
     ) Engine=myisam;
    
    insert into CountryRankBySurfaceAreaPerContinent 
    select Code,case when @prev=Continent then @counter:=@counter + 1 else concat(left(@prev:=Continent,0),@counter:=1) end as Seq 
    from Country 
    order by Continent,Population desc;
    
    select Continent,Name,SurfaceArea,Population 
    from Country c inner join CountryRankBySurfaceAreaPerContinent r on c.Code=r.Code and 6>r.Rank 
    order by Continent,SurfaceArea desc;
    

    This is more like how I would do it. It could even be done without a join by adding a column to the existing table and filling it as shown above.

  12. shlomi wrote:

    @dtecmeister,
    Nice trick. Allow me to point you to two problems, though:
    1. You must have write permissions on that table in order to provide a SELECT query
    2. You must constantly and recurringly execute the query as data changes within the table. Whenever anyone updates/inserts, everything must be re-calculated.

    @shantanu, the above applies to your solution as well.

  13. William wrote:

    Good solution! I would add in the table `tinyint_asc`
    the column `value` as the primary key.

  14. shlomi wrote:

    @William,

    Right you are. I've fixed the code

  15. januzi wrote:

    Hi

    Could You add "explain" result to the last query ?

  16. shlomi wrote:

    +----+-------------+-------------+------+---------------+------+---------+------+------+---------------------------------+
    | id | select_type | table       | type | possible_keys | key  | key_len | ref  | rows | Extra                           |
    +----+-------------+-------------+------+---------------+------+---------+------+------+---------------------------------+
    |  1 | SIMPLE      | Country     | ALL  | NULL          | NULL | NULL    | NULL |  239 | Using temporary; Using filesort |
    |  1 | SIMPLE      | tinyint_asc | ALL  | NULL          | NULL | NULL    | NULL |  256 | Using where; Using join buffer  |
    +----+-------------+-------------+------+---------------+------+---------+------+------+---------------------------------+
    
  17. shlomi wrote:

    Umm, the above is without any keys...

    +----+-------------+-------------+-------+---------------+---------+---------+------+------+-----------------------------------------------------------+
    | id | select_type | table       | type  | possible_keys | key     | key_len | ref  | rows | Extra                                                     |
    +----+-------------+-------------+-------+---------------+---------+---------+------+------+-----------------------------------------------------------+
    |  1 | SIMPLE      | tinyint_asc | range | PRIMARY       | PRIMARY | 1       | NULL |    4 | Using where; Using index; Using temporary; Using filesort |
    |  1 | SIMPLE      | Country     | ALL   | NULL          | NULL    | NULL    | NULL |  239 | Using join buffer                                         |
    +----+-------------+-------------+-------+---------------+---------+---------+------+------+-----------------------------------------------------------+
    
  18. januzi wrote:

    Nice combo in the "extra" column.

  19. shlomi wrote:

    We could do a contest: who gets as many "Extra" comments in a single query :)

    There's no avoiding the "Using join buffer"m as we're essentially joining the tinyint_asc table to Country without an index.
    Here's yet another possible execution plan for the above query.

    +----+-------------+-------------+-------+---------------+---------+---------+------+------+---------------------------------------------+
    | id | select_type | table       | type  | possible_keys | key     | key_len | ref  | rows | Extra                                       |
    +----+-------------+-------------+-------+---------------+---------+---------+------+------+---------------------------------------------+
    |  1 | SIMPLE      | Country     | ALL   | NULL          | NULL    | NULL    | NULL |  239 | Using temporary; Using filesort             |
    |  1 | SIMPLE      | tinyint_asc | range | PRIMARY       | PRIMARY | 1       | NULL |    4 | Using where; Using index; Using join buffer |
    +----+-------------+-------------+-------+---------------+---------+---------+------+------+---------------------------------------------+
    

    This may soften the previous impression.
    We only iterate the Country table once. For each row we join 4 rows from tinyint_asc using join buffer.
    Since we're grouping by columns from two distinct tables, we can't utilize an index to work out the GROUP + ORDER BY.

  20. januzi wrote:

    So, the most reasonable solution is to save the query result in the cache (secondary table, memcached, file, etc). I'll check that last query. Maybe it will give me less than 80k+ Rows_examined (as mysql+percona log patch says in the slow query log).

  21. shlomi wrote:

    @januzi,
    I'm not sure what it is you're trying to avoid or what your situation is.
    The query *requests* that you scan the entire table. It says: "for all rows in the table, group them and sort them within the groups".
    It's true that eventually we then only use top n rows per group. If your groups are small, this makes little difference. If your groups are large, it does.

  22. Keith wrote:

    Hi

    Useful idea.

    One minor suggestion would be to use a table with the numbers 0 to 9 which can then be repeatedly cross joined against itself to produce as big an integer as required.

    Maybe a touch less efficient but more flexible and can then easily be used on several different queries.

  23. shlomi wrote:

    @Keith,
    Yes, that's possible; there's a balance one would want to maintain. It's easy to store 1000 rows in a table, and is faster (verify) than doing 3 joins of 10 rows. Storing 100000 is not as efficient, and joins are required.

  24. MySQL/QueryScript use case: DELETE all but top N records per group | code.openark.org wrote:

    [...] SQL: selecting top N records per group [...]

  25. Erich wrote:

    this is just what i needed - i suggest using char(1) rather the ',' tho

  26. shlomi wrote:

    @Erich,

    Yes, the use of commas assume no commas in values; I also sometimes use char(0) or char(9) which nobody uses within texts

  27. Limit each group in group by | BlogoSfera wrote:

    [...] think one of these solutions could work, but not how: http://code.openark.org/blog/mysql/sql-selecting-top-n-records-per-group [...]

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