distinct snippets

Selecting the "top n" or "n latest" rows by group in Postgres

Tagged postgres, top, latest, window, partition, distinct  Languages sql

This SQL query selects the top 10 transactions for each user:

  • partition the transactions by user_id
  • order the transactions in each partition by the column created_at
  • select only first row, i.e. the latest 
WITH latest_customer_transactions AS (
  SELECT
    *, row_number() OVER (PARTITION BY user_id ORDER BY created_at desc) AS rownum
  FROM
    transactions
)
SELECT
  id, customer_id
FROM
  latest_customer_transactions
WHERE
  rownum <= 10;

Note that if you simply want the first record per group it’s probably best to use DISTINCT ON as described here: https://www.periscopedata.com/blog/first-row-per-group-5x-faster

SELECT DISTINCT ON (customer_id) *
FROM jobs
ORDER BY customer_id,
         priority DESC,
         created_at

Note the differences between RANK, ROW_NUMBER, and DENSE_RANK: http://blog.jooq.org/2014/08/12/the-difference-between-row_number-rank-and-dense_rank/

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