Question

I have a table which contains chronological events. They have a type, time and height.

The data can be summarized like so:

ID, Type, Time, Height  
1, A, time, 0  
2, XX, time, 0   
3, B, time, 3  
4, B, time, 6  
5, C, time, 0  
6, XX, time, 0  
7, A, time, 0  
8, C, time, 0  
9, A, time, 0  
10, B, time, 2  
11, C, time, 0  

etc ( the time column is sorted in ascending order)

i would like to find a SQL statement to list all types of A/B/C where B is the maximum of the height column between types A and C.
So the output would look like:

1, A, time, 0   
4, B, time, 6  
5, C, time, 0  
7, A, time, 0  
8, C, time, 0  
9, A, time, 0  
10, B, time, 2  
11, C, time, 0  

The A/B and C will always be in correct order (i.e. B will always be between A and C), But there may not be a B at all, or there may be multiple B’s between A and C.

The output may/may not list a B event with NULL data if there is no B between A and C.
There is guaranteed to be a C after every A type event.

All XX events shall be ignored in the output. Timestamps over the list will never be duplicated – no two events will contain the same time.

Im guessing to use the MAX function somewhere, and to select all B Rows between A and C depending on the time of A and C.

TIA

Answer 1

Not sure if I have this 100% right, but I find that it’s always best to break this stuff down into smaller queries into temp tables. Here’s a crack at it… (BTW – this is SQL Server T-SQL)

-- get all the type 'a' and type 'c' IDs to represent time spans
if object_id('tempdb..#tab_ac') is not null drop table #tab_ac
select
    a.ID as A_ID,
    (
        select top 1 c.ID
        from tab c
        where c.Time > a.Time
        and c.Type = 'C'
        order by c.Time
    ) as C_ID
into
    #tab_ac
from
    tab a
where
    a.Type = 'A'

create index ix_#tab_ac on #tab_ac (A_ID, C_ID)    

-- get the id with the max height between those spans
if object_id('tempdb..#result1') is not null drop table #result1
select
    ac.*,
    (
        select x.ID
        from tab x
        where x.Time between ta.Time and tc.Time
        order by a.Height desc
    ) as ID_With_Max_Height
into
    #result1
from
    #tab_ac ac join
    tab ta on ac.A_ID = t.ID join
    tab tc on ac.C_ID = t.ID

-- see if that id is type 'B'
select
    *
from
    #result1 r join
    tab t on r.ID_With_Max_Height = t.ID
where
    t.Type = 'B'

Depending on how you want to handle ties for max height, you may want to modify that second query’s ORDER BY clause. Good luck.