Question

I want to create a WINDOW in PostgreSQL and get the row_number() from a number until it appears again. As example, supposing I want to create a window from number 79 until 79 appears again and reset the counting, it must be like this:

number        must be        row_number number
  50                             ?        50
  79                             1        79
  85                             2        85 
  74                             3        74 
  14                             4        14
  79                             1        79
  46                             2        46
  85                             3        85   
  79                             1        79
  45                             2        45  

How I can do this?

Answer 1

Consider this:

-- temporary test table
CREATE TEMP TABLE tbl (id serial, nr int);
INSERT INTO tbl(nr) VALUES
 (50),(79),(85),(74),(14)
,(79),(46),(85),(79),(45);

SELECT id, nr
       ,CASE WHEN grp > 0 THEN
          row_number() OVER (PARTITION BY grp ORDER BY id)::text
        ELSE '?' END AS rn      
FROM  (
    SELECT id, nr
          ,sum(CASE WHEN nr = 79 THEN 1 ELSE 0 END) OVER (ORDER BY id) AS grp
    FROM   tbl) x
-- WHERE grp > 0  -- to exclude numbers before the first 79

Produces exactly your result.