Lag() with condition in sql server

Question

i have a table like this:

Number   Price    Type       Date         Time
------   -----    ----    ----------    ---------
23456    0,665     SV     2014/02/         


        

Answer 1

Your question (and Anon's excellent answer) is part of the SQL of islands and gaps. In this answer, I will try to examine the "row_number() magic" in detail.

I've made a simple example based on events in a ballgame. For each event, we'd like to print the previous and next quarter related message:

create table TestTable (id int identity, event varchar(64));
insert TestTable values
    ('Start of Q1'),
    ('Free kick'),
    ('Goal'),
    ('End of Q1'),
    ('Start of Q2'),
    ('Penalty'),
    ('Miss'),
    ('Yellow card'),
    ('End of Q2');

Here's a query showing off the "row_number() magic" approach:

; with  grouped as
        (
        select  *
        ,       row_number() over (order by id) as rn1
        ,       row_number() over (
                    partition by case when event like '%of Q[1-4]' then 1 end 
                    order by id) as rn2
        from    TestTable
        )
,       order_in_group as
        (
        select  *
        ,       rn1-rn2 as group_nr
        ,       row_number() over (partition by rn1-rn2 order by id) as rank_asc
        ,       row_number() over (partition by rn1-rn2 order by id desc)
                    as rank_desc
        from    grouped
        )
select  *
,       lag(event, rank_asc) over (order by id) as last_event_of_prev_group
,       lead(event, rank_desc) over (order by id) as first_event_of_next_group
from    order_in_group
order by
        id

• The first CTE called "grouped" calculates two row_number()s. The first is 1 2 3 for each row in the table. The second row_number() places pause announcements in one list, and other events in a second list. The difference between the two, rn1 - rn2, is unique for each section of the game. It's helpful to check difference in the example output: it's in the group_nr column. You'll see that each value corresponds to one section of the game.
• The second CTE called "order_in_group" determines the position of the current row within its island or gap. For an island with 3 rows, the positions are 1 2 3 for the ascending order, and 3 2 1 for the descending order.
• Finally, we know enough to tell lag() and lead() how far to jump. We have to lag rank_asc rows to find the final row of the previous section. To find the first row of the next section, we have to lead rank_desc rows.

Hope this helps clarifying the "magic" of Gaps and Islands. Here is a working example at SQL Fiddle.