I have a table like so
Id | Type | Value
--------------------
0 | Big | 2
1 | Big | 3
2 | Small | 3
3 | Small | 3
I would like to get a table like this
Type | Last Value
--------------------
Small | 3
Big | 3
How can I do this. I understand there is an SQL Server method called LAST_VALUE(...) OVER .(..) but I can't get this to work with GROUP BY.
I've also tried using SELECT MAX(ID) & SELECT TOP 1.. but this seems a bit inefficient since there would be a subquery for each value. The queries take too long when the table has a few million rows in it.
Is there a way to quickly get the last value for these, perhaps using LAST_VALUE?
You can do it using rownumber:
select
type,
value
from
(
select
type,
value,
rownumber() over (partition by type order by id desc) as RN
) TMP
where RN = 1
Can't test this now since SQL Fiddle doesn't seem to work, but hopefully that's ok.
The most efficient method might be not exists, which uses an anti-join for the underlying operator:
select type, value
from likeso l
where not exists (select 1 from likeso l2 where l2.type = l.type and l2.id > l.id)
For performance, you want an index on likeso(type, id).
I really wonder if there is more efficent solution but, I use following query on such needs;
Select Id, Type, Value
From ( Select *, Max (Id) Over (Partition By Type) As LastId
From #Table) T
Where Id = LastId
Related
Given the following database table, which records events (status) for different objects (id) with its timestamp:
ID | Date | Time | Status
-------------------------------
7 | 2016-10-10 | 8:23 | Passed
7 | 2016-10-10 | 8:29 | Failed
7 | 2016-10-13 | 5:23 | Passed
8 | 2016-10-09 | 5:43 | Passed
I want to get a result table using plain SQL (MS SQL) like this:
ID | Date | Status
------------------------
7 | 2016-10-10 | Failed
7 | 2016-10-13 | Passed
8 | 2016-10-09 | Passed
where the "status" is the latest entry on a day, given that at least one event for this object has been recorded.
My current solution is using "Outer Apply" and "TOP(1)" like this:
SELECT DISTINCT rn.id,
tmp.date,
tmp.status
FROM run rn OUTER apply
(SELECT rn2.date, tmp2.status AS 'status'
FROM run rn2 OUTER apply
(SELECT top(1) rn3.id, rn3.date, rn3.time, rn3.status
FROM run rn3
WHERE rn3.id = rn.id
AND rn3.date = rn2.date
ORDER BY rn3.id ASC, rn3.date + rn3.time DESC) tmp2
WHERE tmp2.status <> '' ) tmp
As far as I understand this outer apply command works like:
For every id
For every recorded day for this id
Select the newest status for this day and this id
But I'm facing performance issues, therefore I think that this solution is not adequate. Any suggestions how to solve this problem or how to optimize the sql?
Your code seems too complicated. Why not just do this?
SELECT r.id, r.date, r2.status
FROM run r OUTER APPLY
(SELECT TOP 1 r2.*
FROM run r2
WHERE r2.id = r.id AND r2.date = r.date AND r2.status <> ''
ORDER BY r2.time DESC
) r2;
For performance, I would suggest an index on run(id, date, status, time).
Using a CTE will probably be the fastest:
with cte as
(
select ID, Date, Status, row_number() over (partition by ID, Date order by Time desc) rn
from run
)
select ID, Date, Status
from cte
where rn = 1
Do not SELECT from a log table, instead, write a trigger that updates a latest_run table like:
CREATE TRIGGER tr_run_insert ON run FOR INSERT AS
BEGIN
UPDATE latest_run SET Status=INSERTED.Status WHERE ID=INSERTED.ID AND Date=INSERTED.Date
IF ##ROWCOUNT = 0
INSERT INTO latest_run (ID,Date,Status) SELECT (ID,Date,Status) FROM INSERTED
END
Then perform reads from the much shorter lastest_run table.
This will add a performance penalty on writes because you'll need two writes instead of one. But will give you much more stable response times on read. And if you do not need to SELECT from "run" table you can avoid indexing it, therefore the performance penalty of two writes is partly compensated by less indexes maintenance.
I have a table that looks like this:
val | fkey | num
------------------
1 | 1 | 1
1 | 2 | 1
1 | 3 | 1
2 | 3 | 1
What I would like to do is return a set of rows in which values are grouped by 'val', with an array of fkeys, but only where the array of fkeys is greater than 1. So, in the above example, the return would look something like:
1 | [1,2,3]
I have the following query aggregates the arrays:
SELECT val, array_agg(fkey)
FROM mytable
GROUP BY val;
But this returns something like:
1 | [1,2,3]
2 | [3]
What would be the best way of doing this? I guess one possibility would be to use my existing query as a subquery, and do a sum / count on that, but that seems inefficient. Any feedback would really help!
Use Having clause to filter the groups which is having more than fkey
SELECT val, array_agg(fkey)
FROM mytable
GROUP BY val
Having Count(fkey) > 1
Using the HAVING clause as #Fireblade pointed out is probably more efficient, but you can also leverage subqueries:
SQLFiddle: Subquery
SELECT * FROM (
select val, array_agg(fkey) fkeys
from mytable
group by val
) array_creation
WHERE array_length(fkeys,1) > 1
You could also use the array_length function in the HAVING clause, but again, #Fireblade has used count(), which should be more efficient. Still:
SQLFiddle: Having Clause
SELECT val, array_agg(fkey) fkeys
FROM mytable
GROUP BY val
HAVING array_length(array_agg(fkey),1) > 1
This isn't a total loss, though. Using the array_length in the having can be useful if you want a distinct list of fkeys:
SELECT val, array_agg(DISTINCT fkey) fkeys
There may still be other ways, but this method is more descriptive, which may allow your SQL to be easier to understand when you come back to it, years from now.
Having a SQL table, consistent of the columns id and type. I Want to select only the first occurences of a type without using WHERE, since i dont know which types wild occur first, and without LIMIT since i don't know how many.
id | type
---------
1 | 1
2 | 1
3 | 2
4 | 2
5 | 2
E.g.:
SELECT id FROM table ORDER BY type (+ ?) should only return id 1 and 2
SELECT id FROM table ORDER BY type DESC (+ ?) should only return id 3, 4 and 5
Can this be acheived via standard and simple SQL operators?
That's easy. You must use a where clause and evaluate the minimum type there.
SELECT *
FROM mytable
WHERE type = (select min(type) from mytable)
ORDER BY id;
EDIT: Do the same with max() if you want to get the maximum type records.
EDIT: In case the types are not ascending as in your example, you will have to get the type of the minimum/maximum id instead of getting the minimum/maximum type:
SELECT *
FROM mytable
WHERE type = (select type from mytable where id = (select min(id) from mytable))
ORDER BY id;
I have this table:
id | owner | asset | rate
-------------------------
1 | 1 | 3 | 1
2 | 1 | 4 | 2
3 | 2 | 3 | 3
4 | 2 | 5 | 4
And i'm using
SELECT asset, max(rate)
FROM test
WHERE owner IN (1, 2)
GROUP BY asset
HAVING count(asset) > 1
ORDER BY max(rate) DESC
to get intersection of assets for specified owners with best rate.
I also need id of row used for max(rate), but i can't find a way to include it to SELECT. Any ideas?
Edit:
I need
Find all assets that belongs to both owners (1 and 2)
From the same asset i need only one with the best rate (3)
I also need other columns (owner) that belongs to the specific asset with best rate
I expect the following output:
id | asset | rate
-------------------------
3 | 3 | 3
Oops, all 3s, but basically i need id of 3rd row to query the same table again, so resulting output (after second query) will be:
id | owner | asset | rate
-------------------------
3 | 2 | 3 | 3
Let's say it's Postgres, but i'd prefer reasonably cross-DBMS solution.
Edit 2:
Guys, i know how to do this with JOINs. Sorry for misleading question, but i need to know how to get extra from existing query. I already have needed assets and rates selected, i just need one extra field among with max(rate) and given conditions if it's possible.
Another solution that might or might not be faster than a self join (depending on the DBMS' optimizer)
SELECT id,
asset,
rate,
asset_count
FROM (
SELECT id,
asset,
rate,
rank() over (partition by asset order by rate desc) as rank_rate,
count(asset) over (partition by null) as asset_count
FROM test
WHERE owner IN (1, 2)
) t
WHERE rank_rate = 1
ORDER BY rate DESC
You are dealing with two questions and trying to solve them as if they are one. With a subquery, you can better refine by filtering the list in the proper order first (max(rate)), but as soon as you group, you lose this. As such, i would set up two queries (same procedure, if you are using procedures, but two queries) and ask the questions separately. Unless ... you need some of the information in a single grid when output.
I guess the better direction to head is to have you show how you want the output to look. Once you bake the input and the output, the middle of the oreo is easier to fill.
SELECT b.id, b.asset, b.rate
from
(
SELECT asset, max(rate) maxrate
FROM test
WHERE owner IN (1, 2)
GROUP BY asset
HAVING count(asset) > 1
) a, test b
WHERE a.asset = b.asset
AND a.maxrate = b.rate
ORDER BY b.rate DESC
You don't specify what type of database you're running on, but if you have analytical functions available you can do this:
select id, asset, max_rate
from (
select ID, asset, max(rate) over (partition by asset) max_rate,
row_number() over (partition by asset order by rate desc) row_num
from test
where owner in (1,2)
) q
where row_num = 1
I'm not sure how to add in the "having count(asset) > 1" in this way though.
This first searches for rows with the maximum rate per asset. Then it takes the highest id per asset, and selects that:
select *
from test
inner join
(
select max(id) as MaxIdWithMaxRate
from test
inner join
(
select asset
, max(rate) as MaxRate
from test
group by
asset
) filter
on filter.asset = test.asset
and filter.MaxRate = test.rate
group by
asset
) filter2
on filter.MaxIdWithMaxRate = test.id
If multiple assets share the maximum rate, this will display the one with the highest id.
Is there a way to select records based using an if statement?
My table looks like this:
id | num | dis
1 | 4 | 0.5234333
2 | 4 | 8.2234
3 | 8 | 2.3325
4 | 8 | 1.4553
5 | 4 | 3.43324
And I want to select the num and dis where dis is the lowest number... So, a query that will produce the following results:
id | num | dis
1 | 4 | 0.5234333
4 | 8 | 1.4553
If you want all the rows with the minimum value within the group:
SELECT id, num, dis
FROM table1 T1
WHERE dis = (SELECT MIN(dis) FROM table1 T2 WHERE T1.num = T2.num)
Or you could use a join to get the same result:
SELECT T1.id, T1.num, T1.dis
FROM table1 T1
JOIN (
SELECT num, MIN(dis) AS dis
FROM table1
GROUP BY num
) T2
ON T1.num = T2.num AND T1.dis = T2.dis
If you only want a single row from each group, even if there are ties then you can use this:
SELECT id, dis, num FROM (
SELECT id, dis, num, ROW_NUMBER() OVER (PARTITION BY num ORDER BY dis) rn
FROM table1
) T1
WHERE rn = 1
Unfortunately this won't be very efficient. If you need something more efficient then please see Quassnoi's page on selecting rows with a groupwise maximum for PostgreSQL. Here he suggests several ways to perform this query and explains the performance of each. The summary from the article is as follows:
Unlike MySQL, PostgreSQL implements
several clean and documented ways to
select the records holding group-wise
maximums, including window functions
and DISTINCT ON.
However to the lack of the loose index
scan support by the PostgreSQL’s
optimizer and the less efficient usage
of indexes in PostgreSQL, the queries
using these function take too long.
To work around these problems and
improve the queries against the low
cardinality grouping conditions, a
certain solution described in the
article should be used.
This solution uses recursive CTE’s to
emulate loose index scan and is very
efficient if the grouping columns have
low cardinality.
Use this:
SELECT DISTINCT ON (num) id, num, dis
FROM tbl
ORDER BY num, dis
Or if you intend to use other RDBMS in future, use this:
select * from tbl a where dis =
(select min(dis) from tbl b where b.num = a.num)
If you need to have IF logic you can use PL/pgSQL.
http://www.postgresql.org/docs/8.4/interactive/plpgsql-control-structures.html
But try to solve your issue with SQL first if possible, it will be faster and use PL/pgSQL when SQL can't solve your problem.