I'm stuck with how to write SQL statements, so I would appreciate it if you could teach me.
Current status
items table
id
session_id
item_id
competition_id
1
1
2
1
2
1
3
1
2
1
2
1
2
1
2
1
2
1
5
2
3
1
7
2
4
1
4
2
5
1
5
2
want to
grouping by competition_id,
Count the same numbers in item_id,Extract the most common numbers and their numbers.
For example
If competition_id is 1,item_id → 2 ,and the number is 3
If competition_id is 2,item_id → 5 ,and the number is 2
If competition_id is 3,・・・
If competition_id is 4,・・・
environment
macOS BigSur
ruby 2.7.0
Rails 6.1.1
sqlite
In statistics, what you are asking for is the mode, the most common value.
You can use aggregation and row_number():
select ct.*
from (select competition_id, item_id, count(*) as cnt,
row_number() over (partition by competition_id order by count(*) desc) as seqnum
from t
group by competition_id, item_id
) ci
where seqnum = 1;
In the event that there are ties, this returns only one of the values, arbitrarily. If you want all modes when there are ties use rank() instead of row_number().
Related
I've got this table ratings:
id
user_id
type
value
0
0
Rest
4
1
0
Bar
3
2
0
Cine
2
3
0
Cafe
1
4
1
Rest
4
5
1
Bar
3
6
1
Cine
2
7
1
Cafe
5
8
2
Rest
4
9
2
Bar
3
10
3
Cine
2
11
3
Cafe
5
I want to have a table with a row for every pair (user_id, type) for the top 3 rated types through all users (ranked by sum(value) across the whole table).
Desired result:
user_id
type
value
0
Rest
4
0
Cafe
1
0
Bar
3
1
Rest
4
1
Cafe
5
1
Bar
3
2
Rest
4
3
Cafe
5
2
Bar
3
I was able to do this with two queries, one to get the top 3 and then another to get the rows where the type matches the top 3 types.
Does someone know how to fit this into a single query?
Get rows per user for the 3 highest ranking types, where types are ranked by the total sum of their value across the whole table.
So it's not exactly about the top 3 types per user, but about the top 3 types overall. Not all users will have rows for the top 3 types, even if there would be 3 or more types for the user.
Strategy:
Aggregate to get summed values per type (type_rnk).
Take only the top 3. (Break ties ...)
Join back to main table, eliminating any other types.
Order result by user_id, type_rnk DESC
SELECT r.user_id, r.type, r.value
FROM ratings r
JOIN (
SELECT type, sum(value) AS type_rnk
FROM ratings
GROUP BY 1
ORDER BY type_rnk DESC, type -- tiebreaker
LIMIT 3 -- strictly the top 3
) v USING (type)
ORDER BY user_id, type_rnk DESC;
db<>fiddle here
Since multiple types can have the same ranking, I added type to the sort order to break ties alphabetically by their name (as you did not specify otherwise).
Turns out, we don't need window functions - the ones with OVER and, optionally, PARTITION for this. (Since you asked in a comment).
I think you just want row_number(). Based on your results, you seem to want three rows per type, with the highest value:
select t.*
from (select t.*,
row_number() over (partition by type order by value desc) as seqnum
from t
) t
where seqnum <= 3;
Your description suggests that you might just want this per user, which is a slight tweak:
select t.*
from (select t.*,
row_number() over (partition by user order by value desc) as seqnum
from t
) t
where seqnum <= 3;
ID UserID LevelID
1 1 1
2 1 2
3 1 2
4 1 2
5 2 1
6 2 3
7 3 2
8 4 1
9 4 1
The query should return: LevelID: 1 (3 times) - the LevelID column that is most frequently repeated by different Users (UserID).
I have the following query:
SELECT LevelID, COUNT(LevelID) AS 'Occurrence'
FROM
(
SELECT DISTINCT * FROM
(
SELECT UserID, LevelID
FROM SampleTable
) cv
) levels
GROUP BY LevelID
ORDER BY 'Occurrence' DESC
Which returns:
LevelID Occurence
1 3
2 2
3 1
But it doesn't let me to add LIMIT 1; at the bottom to retrieve the first top row of the selection. What's wrong with the query?
There is no need for these several levels of nesting. Consider using aggregation, count(distinct ...), ordering the results and using a row-limiting clause to keep the top record only:
select top(1) levelID, count(distinct userID) cnt
from mytable
group by levelID
order by cnt desc
If you want to allow possible top ties, then use top (1) with ties instead of just top (1).
Consider the following table:
ID GroupId Rank
1 1 1
2 1 2
3 1 1
4 2 10
5 2 1
6 3 1
7 4 5
I need an sql (for MS-SQL) select query selecting a single Id for each group with the lowest rank. Each group needs to only return a single ID, even if there are two with the same rank (as 1 and 2 do in the above table). I've tried to select the min value, but the requirement that only one be returned, and the value to be returned is the ID column, is throwing me.
Does anyone know how to do this?
Use row_number():
select t.*
from (select t.*,
row_number() over (partition by groupid order by rank) as seqnum
from t
) t
where seqnum = 1;
I have the table below in an SQL database.
user rating
1 10
1 7
1 6
1 2
2 8
2 3
2 2
2 2
I would like to keep only the best two ratings by user to get:
user rating
1 10
1 7
2 8
2 3
What would be the SQL query to do that? I am not sure how to do it.
It will work
;with cte as
(select user,rating, row_number() over (partition by user order by rating desc) maxval
from yourtable)
select user,rating
from cte
where maxval in (1,2)
I have a table containing items in a priority order as such:
id priority
1 1
2 2
3 3
4 8
5 3
6 4
Currently I retrieve items (SQL Server) in priority order, although a random item when there are matching priorities using the following query:
select item
from table
order by priority, newid()
This will return
id priority
1 1
2 2
3 3
5 3
6 4
4 8
or
id priority
1 1
2 2
5 3
3 3
6 4
4 8
So it's approximately 50/50 traffic
I now have a requirement to only retrieve one row of the rows when there are two matching priorities, for example..
id priority
1 1
2 2
3 3
6 4
4 8
or
id priority
1 1
2 2
5 3
6 4
4 8
You can use ROW_NUMBER, presuming SQL-Server (because of NEWID):
WITH CTE AS
(
SELECT t.*, RN = ROW_NUMBER() OVER (PARTITION BY Priority
ORDER BY ID)
FROM dbo.table t
)
SELECT * FROM CTE WHERE RN = 1
If these are all columns you could also use this sql:
SELECT MIN(t.ID) AS ID, t.Priority
FROM dbo.table t
GROUP BY t.priority
Update "No, I need to be able to get a random row when two (or more) priorities match"
Then i have misunderstood your requirement. You can use ORDER BY NEWID:
WITH CTE AS
(
SELECT t.*, RN = ROW_NUMBER() OVER (PARTITION BY Priority
ORDER BY NEWID())
FROM dbo.table t
)
SELECT * FROM CTE WHERE RN = 1