Using LAG function to find a previous record

Question

Hey,
I tried searching the forum for this, but I actually didn't even know what to search for, so I'm creating a new thread.
The SQL below displays a list of prices:
WITH T AS (
  SELECT 1  AS ID, 'PERM' AS TYPE, 100 AS PRICE, SYSDATE + 1 AS START_DATE FROM DUAL UNION
  SELECT 3  AS ID, 'TEMP' AS TYPE, 90  AS PRICE, SYSDATE + 2 AS START_DATE FROM DUAL UNION
  SELECT 7  AS ID, 'TEMP' AS TYPE, 80  AS PRICE, SYSDATE + 3 AS START_DATE FROM DUAL UNION
  SELECT 8  AS ID, 'PERM' AS TYPE, 75  AS PRICE, SYSDATE + 4 AS START_DATE FROM DUAL UNION
  SELECT 16 AS ID, 'TEMP' AS TYPE, 70  AS PRICE, SYSDATE + 5 AS START_DATE FROM DUAL UNION
  SELECT 20 AS ID, 'TEMP' AS TYPE, 60  AS PRICE, SYSDATE + 6 AS START_DATE FROM DUAL UNION
  SELECT 34 AS ID, 'TEMP' AS TYPE, 50  AS PRICE, SYSDATE + 7 AS START_DATE FROM DUAL
  SELECT T.ID
       , T.TYPE
       , T.PRICE
       , TRUNC (T.START_DATE) AS START_DATE
       , CASE
           WHEN T.TYPE = 'PERM'
             THEN T.ID
             ELSE LAG (T.ID, 1, NULL) OVER (PARTITION BY NULL ORDER BY T.ID)
         END AS BASE_ID
    FROM T
ORDER BY T.START_DATEThe challenge is to produce this output:
ID TYPE PRICE BASE_ID
1 PERM   100       1
3 TEMP    90       1
7 TEMP    80       1
8 PERM    75       8
16 TEMP    70       8
20 TEMP    60       8
34 TEMP    50       8What I want to achieve is to bring a column with the ID of the most recent PERM price for TEMP prices,
and It's own ID for PERM prices.
My attempt uses LAG to navigate back on the record set, but it uses 1 statically. If there was a way to come up with a number
for each TEMP price saying how far it was from the most recent PERM, then I could use that number instead of 1.
Something like:
ID TYPE PRICE DISTANCE_FROM_PREV_PERM
1 PERM   100                       0
3 TEMP    90                       1
7 TEMP    80                       2
8 PERM    75                       0
16 TEMP    70                       1
20 TEMP    60                       2
34 TEMP    50                       3Any help will be greatly appreciated.
Thanks.

Answer 1

Maybe
select id,type,price,
       last_value(base_id) ignore nulls over (order by the_row) base_id
/*     last_value(base_id ignore nulls) over (order by the_row) base_id  -- old way */
  from (select id,type,price,start_date,
               case type when 'PERM' then id end base_id,
               row_number() over (order by start_date) the_row
          from t
       )Regards
Etbin