How to select most frequent value in a column per each id group?

Question

I have a table in SQL that looks like this:

user_id | data1
0       | 6
0       | 6
0       | 6
0       | 1
0       | 1
0       | 2
1       | 5
1       | 5
1         


        

Answer 1

If you use proper "order by" then distinct on (user_id) make the same work because it takes 1.line from data partitioned by "user_id". DISTINCT ON is specialty of PostgreSQL.

select distinct on (user_id) user_id, most_frequent_value from (
SELECT user_id, data1 AS most_frequent_value, count(*) as _count
FROM my_table
GROUP BY user_id, data1) a
ORDER BY user_id, _count DESC