SQL Occurrence Comparison? - mysql

Question

**Above is a picture of this particular table*
I need to write a query for a database that lists the name of a department for the department that controls the most projects.
In my database, departments are identified by dnums.
So my question is, how can I write something that checks for the greatest occurrence of a Dnum in SQL? Because that's how I will identify the department that controls the most projects.
I've tried several different queries, but none of them work properly.
Could anyone explain a method that could compare occurrences?

Answer 1

You know already how to count per department:
select dnum, count(*) from project group by dnum;
In SQL Server it is easy to select the dnum(s) with the maximum occurrences; you order by count descending and take the top row(s) using TOP() WITH TIES.
select top(1) with ties dnum from project group by dnum order by count(*) desc;
(In standard SQL that would be order by count(*) desc fetch 1 row with ties).
In standard SQL (and SQL Server) you also have the option of ranking your records per count:
select dnum
from (select dnum, rank() over (order by count(*) desc) as rnk from project) ranked
where rnk = 1;
MySQL doesn't give you any of these options, lacking both a WITH TIES clause and analytic functions such as RANK.
So in MySQL you would not find the departments with the maximum count in one step, but only the maximum count alone first. You would get the according department(s) only in a second step.
Two approaches here:
select count(*) from project group by dnum order by count(*) desc limit 1;
or
select max(cnt) from (select count(*) as cnt from project group by dnum) counted;
Then join the counted departments again:
select p.dnum
from
(
select count(*) as cnt
from project
group by dnum
order by count(*) desc limit 1
) m
(
select dnum, count(*) as cnt
from project
group by dnum
) p on p.cnt = m.cnt;
The last step is the same in both DBMS:
select dname
from departments
where dnumber in (select dnum ...);
(Or join the departments table instead, so you can show both name and count.)

Answer 2

You can use the COUNT function:
SELECT dnum, COUNT(*)
FROM project
GROUP BY dnum
ORDER BY COUNT(*)
If you need the department_name, you'll have to join to your department table (assuming you have one). It could look something like this:
SELECT d.dnum, d.name, COUNT(p.pnumber)
FROM department d
INNER JOIN projects p ON d.dnum = p.dnum
GROUP BY d.dnum, d.name
ORDER BY COUNT(p.pnumber)