How to select all records of n groups? - sql

Question

I want to select the records of the top n groups. My data looks like this:
Table 'runner':
id gid status rtime
---------------------------
100 5550 1 2016-08-19
200 5550 2 2016-08-22
300 5550 1 2016-08-30
100 6050 3 2016-09-01
200 6050 1 2016-09-02
100 6250 1 2016-09-11
200 6250 1 2016-09-15
300 6250 3 2016-09-19
Table 'static'
id description env
-------------------------------
100 something 1 somewhere 1
200 something 2 somewhere 2
300 something 3 somewhere 3
The unit id (id) is unique within the group but not unique in its column, because an instance of the group is generated regularly. The group id (gid) is assigned to every unit but will not generate on more than one instance.
Now, combining the tables and selecting everything or filter by a specific value is easy, but how do I select all records of, for example, the first two groups without directly refering to the group ids?
Expected result would be:
id gid description status rtime
--------------------------------------
300 6250 something 2 3 2016-09-19
200 6250 something 1 1 2016-09-15
100 6250 something 3 1 2016-09-11
200 6050 something 2 1 2016-09-02
100 6050 something 1 3 2016-09-01
Extra Question: When I filter for a timeframe like this:
[...]
WHERE runner.rtime BETWEEN '2016-08-25' AND '2016-09-16'
Is there a simple way of ensuring, that groups are not cut off but either appear with all their records or not at all?

Answer 1

You can use a ROW_NUMBER() to do this. First, create a query to rank groups:
SELECT gid, ROW_NUMBER() over (order by gid desc) as RN
FROM Runner
GROUP BY gid
Then use this as a derived table to get your other info, and use a where clause to filter to the number of groups you want to see. For instance, the below would return the top 5 groups RN <= 5:
SELECT id, R.gid, description, status, rtime
FROM (SELECT gid, ROW_NUMBER() over (order by gid desc) as RN
FROM Runner
GROUP BY gid) G
INNER JOIN Runner R on R.gid = G.gid
INNER JOIN Statis S on S.id = R.id
WHERE RN <= 5 --Change this to see more or less groups
For your second question about dates, you can do this with a subquery like so:
SELECT *
FROM Runner
WHERE gid IN (SELECT gid
FROM Runner
WHERE rtime BETWEEN '2016-08-25' AND '2016-09-16')

Answer 2

Hmmm. I suspect this might do what you want:
select top (1) with ties r.*
from runner r
order by min(rtime) over (partition by gid), gid;
At least, this will get the complete first group.
In any case, the idea is to include gid as a key in the order by and to use top with ties.

Answer 3

you can do the following
with report as(
select n.id,n.gid,m.description,n.status,n.rtime, dense_rank() over(order by gid desc) as RowNum
from #table1 n
inner join #table2 m on n.id = m.id )
select id,gid,description,status,rtime
from report
where RowNum<=2 -- <-- here n=2
order by gid desc,rtime desc
here a working demo

Answer 4

DENSE_RANK looks like a ideal solution here
Select * From
(
select DENSE_RANK() over (order by gid desc) as D_RN, r.*
from runner r
) A
Where D_RN = 1

Answer 5

No need to use ranking functions (ROW_NUMBER, DENSE_RANK etc).
SELECT r.id, gid, [description], [status], rtime
FROM runner r
INNER JOIN static s ON r.id = s.id
WHERE gid IN (
SELECT TOP 2 gid FROM runner GROUP BY gid ORDER BY gid DESC
)
ORDER BY rtime DESC;
The same using CTE:
WITH grouped
AS
(
SELECT TOP 2 gid
FROM runner GROUP BY gid ORDER BY gid DESC
)
SELECT r.id, grouped.gid, [description], [status], rtime
FROM runner r
INNER JOIN static s ON r.id = s.id
INNER JOIN grouped ON r.gid = grouped.gid
ORDER BY rtime DESC;