SQL - Displaying entries that are the max of a count? - sql

Question

CREATE TABLE doctor( patient CHAR(13), docname CHAR(30) );
Say I had a table like this, then how would I display the names of the doctors that have the most patients? Like if the most was three and two doctors had three patients then I would display both of their names.
This would get the max patients:
SELECT MAX(count)
FROM (SELECT COUNT(docname) FROM doctor GROUP BY docname) a;
This is all the doctors and how many patients they have:
SELECT docname, COUNT(docname) FROM doctor GROUP BY name;
Now I can't figure out how to combine them to list only the names of doctors who have the max patients.
Thanks.

Answer 1

This should do it.
SELECT docname, COUNT(*) FROM doctor GROUP BY name HAVING COUNT(*) =
(SELECT MAX(c) FROM
(SELECT COUNT(patient) AS c
FROM doctor
GROUP BY docname))
On the other hand if you require only the first entry, then
SELECT docname, COUNT(docname) FROM doctor
GROUP BY name
ORDER BY COUNT(docname) DESC LIMIT 1;

Answer 2

This should do it for you:
SELECT docname
FROM doctor
GROUP BY docname
HAVING COUNT(patient)=
(SELECT MAX(patientcount) FROM
(SELECT docname,COUNT(patient) AS patientcount
FROM doctor
GROUP BY docname) t1)

Answer 3

Here's another alternative that only has one subquery instead of two:
SELECT docname
FROM author
GROUP BY name
HAVING COUNT(*) = (
SELECT COUNT(*) AS c
FROM author
GROUP BY name
ORDER BY c DESC
LIMIT 1
)

Answer 4

Allowing for any feature in any ISO SQL specification since you did not specify a database product or version, and assuming that the table of patients is called "patients" and has a column called "docname", the following might give you what you wanted:
With PatientCounts As
(
Select docname
, Count(*) As PatientCount
From patient
Group By docname
)
, RankedCounts As
(
Select docname, PatientCount
, Rank() Over( Order By PatientCount ) As PatientCountRank
From PatientCounts
)
Select docname, PatientCount, PatientCountRank
From RankedCounts
Where PatientCountRank = 1

Answer 5

While using ... HAVING COUNT(*) = ( ...MAX().. ) works:
Within the query, it needs almost the same sub-query twice.
For most databases, it needs a 2nd level sub-query as MAX( COUNT(*) )
is not supported.
While using TOP / LIMIT / RANK etc works:
It uses SQL extensions for a specific database.
Also, using TOP / LIMIT of 1 will only give one row - what if there are two or more doctors with the same maximum number of patients?
I would break the problem into steps:
Get target field(s) and associated count
SELECT docName, COUNT( patient ) AS countX
FROM doctor
GROUP BY docName
Using the above as a 'statement scoped view', use this to get the max count row(s)
WITH x AS
(
SELECT docName, COUNT( patient ) AS countX
FROM doctor
GROUP BY docName
)
SELECT x.docName, x.countX
FROM x
WHERE x.countX = ( SELECT MAX( countX ) FROM x )
The WITH clause, which defines a 'statement scoped view', effectively gives named sub-queries that can be re-used within the same query.
While this solution, using statement scoped views, is longer, it is:
Easier to test
Self documenting
Extendable
It is easier to test as parts of the query can be run standalone.
It is self documenting as the query directly reflects the requirement
ie the statement scoped view lists the target field(s) and associated count.
It is extendable as if other conditions or fields are required, this can be easily added to the statement scoped view.
eg in this case, the table stucture should be changed to include a doctor-id as a primary key field and this should be part of the results.

Answer 6

Take both queries and join them together to get the max:
SELECT
docName,
m.MaxCount
FROM
author
INNER JOIN
(
SELECT
MAX(count) as MaxCount,
docName
FROM
(SELECT
COUNT(docname)
FROM
doctor
GROUP BY
docname
)
) m ON m.DocName = author.DocName

Answer 7

Another alternative using CTE:
with cte_DocPatients
as
(
select docname, count(*) as patientCount
from doctor
group by docname
)
select docname, patientCount from
cte_DocPatients where
patientCount = (select max(patientCount) from cte_DocPatients)

Answer 8

if you do not need to care about performance I think just sorting and pick first element. Something like this:
SELECT docname, COUNT(docname) as CNT
FROM doctor
WHERE docname = docname
GROUP BY docname
ORDER BY CNT DESC
LIMIT 1

Answer 9

This will give you each doctor name and respective count of treating patients
SELECT docname, COUNT(docname) as TreatingPatients FROM doctor
WHERE docname = docname
GROUP BY docname