Sql – Get most common value for each value of another column in SQL

greatest-n-per-grouppostgresqlsql

I have a table like this:

 Column  | Type | Modifiers 
---------+------+-----------
 country | text | 
 food_id | int  | 
 eaten   | date | 

And for each country, I want to get the food that is eaten most often. The best I can think of (I'm using postgres) is:

CREATE TEMP TABLE counts AS 
   SELECT country, food_id, count(*) as count FROM munch GROUP BY country, food_id;

CREATE TEMP TABLE max_counts AS 
   SELECT country, max(count) as max_count FROM counts GROUP BY country;

SELECT country, max(food_id) FROM counts 
   WHERE (country, count) IN (SELECT * from max_counts) GROUP BY country;

In that last statement, the GROUP BY and max() are needed to break ties, where two different foods have the same count.

This seems like a lot of work for something conceptually simple. Is there a more straight forward way to do it?

Best Answer

It is now even simpler: PostgreSQL 9.4 introduced the mode() function:

select mode() within group (order by food_id)
from munch
group by country

returns (like user2247323's example):

country | mode
--------------
GB      | 3
US      | 1

See documentation here: https://wiki.postgresql.org/wiki/Aggregate_Mode

https://www.postgresql.org/docs/current/static/functions-aggregate.html#FUNCTIONS-ORDEREDSET-TABLE