Given the following table Attendance, where AttID is the primary key and table is sorted. I'm attempting to search in the index of MemberID in each ClassID, or return total members in a class if the MemberID does not exist in the class (this condition is less important to me).
AttID
ClassID
MemberID
1
1
1
2
1
2
3
1
3
4
2
30
5
2
40
5
2
1
6
2
50
For example:
Given the target MemberID is 1, I will get the following
ClassID
Index
1
1
2
3
Given the target MemberID is 2, I will get the following
ClassID
Index
1
2
2
4
I'm using these results to determine whether a member that attended a class is within the classes' capacity.
"5" is repeated for Attid so it is not a primary key. I will assume this is a typo.
You have basically described the row_number() function:
select a.*
from (select a.*,
row_number() over (partition by classid order by attid) as seqnum
from attendance a
) a
where memberid = ?
Related
Let's suppose that I have this simple table:
ID
1
2
3
4
If I do:
WITH example AS
(
SELECT UNNEST(ARRAY[1,2,3,4]) as ID
)
SELECT ID, lAG(ID,1) over() as LAG_ID
FROM example
-- The shift is 1 in this case, but could be any integer in practice.
I got:
ID
LAG_ID
1
2
1
3
2
4
3
But I need a circular shift:
ID
LAG_ID
1
4
2
1
3
2
4
3
Is there an elegant way to produce this result ?
If ID is not nullable
WITH example AS
(
SELECT UNNEST(ARRAY[1,2,3,4]) as ID
)
SELECT ID, coalesce(lAG(ID,1) over(order by ID asc), (select max(ID) from example)) as LAG_ID
FROM example
I have an existing table to which I have added a new column which is supposed to hold the Id of a record in another (new) table.
Simplified structure is sort of like this:
Customer table
[CustomerId] [GroupId] [LicenceId] <-- new column
Licence table <-- new table
[LicenceId] [GroupId]
The Licence table has a certain number of licences per group than can be assigned to customers in that same group. There are multiple groups, and each group has a variable number of customers and licences.
So say there are 100 licences available for group 1 and there are 50 customers in group 1, so each can get a license. There are never more customers than there are licences.
Sample
Customer
[CustomerId] [GroupId] [LicenceId]
1 1 NULL
2 1 NULL
3 1 NULL
4 1 NULL
5 2 NULL
6 2 NULL
7 2 NULL
8 3 NULL
9 3 NULL
Licence
[LicenceId] [GroupId]
1 1
2 1
3 1
4 1
5 1
6 1
7 2
8 2
9 2
10 2
11 2
12 3
13 3
14 3
15 3
16 3
17 3
Desired outcome
Customer
[CustomerId] [GroupId] [LicenceId]
1 1 1
2 1 2
3 1 3
4 1 4
5 2 7
6 2 8
7 2 9
8 3 12
9 3 13
So now I have to do this one time update to give every customer a licence and I have no idea how to go about it.
I'm not allowed to use a cursor. I can't seem to do a MERGE UPDATE, because joining the Customer to the Licence table by GroupId will result in multiple hits.
How do I assign each customer the next available LicenceId within their group in one query?
Is this even possible?
You can use window functions:
with c as (
select c.*, row_number() over (partition by groupid order by newid()) as seqnum
from customers c
),
l as (
select l.*, row_number() over (partition by groupid order by newid()) as seqnum
from licenses c
)
update c
set c.licenceid = l.licenseid
from c join
l
on c.seqnum = l.seqnum and c.groupid = l.groupid;
This assigns the licenses randomly. That is really just for fun. The most efficient method is to use:
row_number() over (partition by groupid order by (select null)) as seqnum
SQL Server often avoids an additional sort operation in this case.
But you might want to order them by something else -- for instance by the ordering of the customer ids, or by some date column, or something else.
Gordon has put it very well in his answer.
Let me break it down into simpler steps for you.
Step 1. Use the ROW_NUMBER() function to assign a SeqNum to the Customers. Use PARTITION BY GroupId so that the number starts from 1 in every group. I would ORDER BY CustomerId
Step 2. Use the ROW_NUMBER() function to assign a SeqNum to the Licences. Use PARTITION BY GroupId so that the number starts from 1 in every group. ORDER BY LicenseId because your ask is to "assign each customer the next available LicenceId within their group".
Now use these 2 queries to update LicenseId in Customer table.
It's my first question here so I hope I can explain it well enough,
I want to order my data by amount of occurrences in the table.
My table is like this:
id Daynr
1 2
1 4
2 4
2 5
2 6
3 1
4 2
4 5
And I want it to sort it like this:
id Daynr
3 1
1 2
1 4
4 2
4 5
2 4
2 5
2 6
Player #3 has one day in the table, and Player #1 has 2.
My table is named "dayid"
Both id and Daynr are foreign keys, together making it a primary key
I hope this explains my problem enough, Please ask for more information it's my first time here.
Thanks in advance
You can do this by counting the number of times that things occur for each id. Most databases support window functions, so you can do this as:
select id, daynr
from (select t.*, count(*) over (partition by id) as cnt
from table t
) t
order by cnt, id;
You can also express this as a join:
select t.id, t.daynr
from table as t inner join
(select id, count(*) as cnt
from table
group by id
) as tg
on t.id = tg.id
order by tg.cnt, id;
Note that both of these include the id in the order by. That way, if two ids have the same count, all rows for the id will appear together.
I have a single table, where I want to return a list of the MAX(id) GROUPed by another identifier. However I have a third column that, when it meets a certain criteria, "trumps" rows that don't meet that criteria.
Probably easier to explain with an example. Sample table has:
UniqueId (int)
GroupId (int)
IsPriority (bit)
Raw data:
UniqueId GroupId IsPriority
-----------------------------------
1 1 F
2 1 F
3 1 F
4 1 F
5 1 F
6 2 T
7 2 T
8 2 F
9 2 F
10 2 F
So, because no row in groupId 1 has IsPriority set, we return the highest UniqueId (5). Since groupId 2 has rows with IsPriority set, we return the highest UniqueId with that value (7).
So output would be:
5
7
I can think of ways to brute force this, but I am looking to see if I can do this in a single query.
SQL Fiddle Demo
WITH T
AS (SELECT *,
ROW_NUMBER() OVER (PARTITION BY GroupId
ORDER BY IsPriority DESC, UniqueId DESC ) AS RN
FROM YourTable)
SELECT UniqueId,
GroupId,
IsPriority
FROM T
WHERE RN = 1
What's the best way to get a sublist of things?
I have two tables:
create table A (
id int primary key
)
create table B (
id int primary key,
aid int foreign key references A( id ),
sort_key int
)
I want to get a list of objects A, with subobjects B, but only the top five of B.
Let's say A is people, and B is type of food, with sort_key being how much a person likes that food. How do I get everybody (or some people) and their top 5 food?
On the previous comment if it's an INT you can't put non numerics in there.
Assuming the following data:
a
--
id
1
2
3
b
------------------------
id aid sort_key
1 1 1
2 1 2
3 2 1
4 3 1
5 1 3
6 1 4
7 1 5
8 1 6
9 2 2
10 2 3
The following query in MySQL would give you this:
SELECT a.*,
(SELECT GROUP_CONCAT(id) AS ids FROM b AS bi WHERE bi.aid = a.id ORDER BY sort_key LIMIT 5) AS ids
FROM a
Result:
id ids
1 1,2,5,6,7,8
2 3,9,10
3 4
This query assumes the sort key is one based, rather than zero:
SELECT a.name
b.food
FROM A a
JOIN B b ON b.aid = a.id
WHERE b.sortkey <= 5
ORDER BY a.name, b.sortkey