PostgreSQL equivalente per Top N con cravatte: limite "con legami"?

Question

Sto cercando qualcosa simile a questo in SQL Server:

SELECT TOP n WITH TIES FROM tablename

.

Conosco il LIMIT in PostgreSQL, ma esiste l'equivalente di quanto sopra?Sono solo curioso di salvare una query extra ogni volta per me.

Se ho una tabella Numbers con Attribute nums: {10, 9, 8, 8, 2}.Voglio fare qualcosa come:

SELECT nums FROM Numbers ORDER BY nums DESC LIMIT *with ties* 3

.

Dovrebbe restituire {10, 9, 8, 8} perché ci vuole il top 3 più il 8 extra poiché lega l'altro.

Answer 1

There is no WITH TIES clause in PostgreSQL like there is in SQL Server.
In PostgreSQL I would substitute this for TOP n WITH TIES .. ORDER BY <something>:

WITH cte AS (
   SELECT *, rank() OVER (ORDER BY <something>) AS rnk
   FROM   tbl
   )
SELECT *
FROM   cte
WHERE  rnk <= n;

To be clear, rank() is right, dense_rank() would be wrong (return too many rows).
Consider this quote from the SQL Server docs (from the link above):

For example, if expression is set to 5 but 2 additional rows match the values of the ORDER BY columns in row 5, the result set will contain 7 rows.

The job of WITH TIES is to include all peers of the last row in the top n as defined by the ORDER BY clause. rank() gives the exact same result.

To make sure, I tested with SQL server, here is a live demo.
And here is a more convenient SQLfiddle.

Answer 2

Try this:

Output: 10, 9, 8, 8

with numbers (nums) as (
  values (10), (9), (8), (8), (2)
) 
SELECT nums FROM Numbers 
WHERE nums in (SELECT DISTINCT nums FROM Numbers ORDER BY nums DESC LIMIT 3)
ORDER BY nums DESC

Output: 10,10,9,8,8

with numbers (nums) as (
  values (10), (9), (8), (8), (2), (10)
) 
SELECT nums FROM Numbers 
WHERE nums in (SELECT DISTINCT nums FROM Numbers ORDER BY nums DESC LIMIT 3)
ORDER BY nums DESC