SQL - Retrieving data within groups before and after some condition - sql

Question

With the two following tables:
EMPLOYEE (Fname, Lname, SSN, DNO)
DEPARTMENT (Dname, Dnumber)
For each department that has more than five employees, retrieve the
department name and the number of its employees who are making more
than $40,000
Here is an incorrect solution to this:
SELECT
dname,
COUNT(*)
FROM
Department, Employee
WHERE
dnumber = dno
AND salary > 40000
GROUP BY
dname
HAVING
COUNT(*) > 5;
It is clear that it would not list any department that have five or more employees unless they all have more than $40,000 salary, because where is applied before group by clause. which is not what we want.
Here is the correct solution:
SELECT
dname, COUNT(*)
FROM
Department, Employee
WHERE
dnumber = dno
AND salary > 40000
AND dno IN (SELECT dno
FROM Employee
GROUP BY dno
HAVING COUNT(*) > 5)
GROUP BY
dname
I cant see why is this correct?
Isn't it going to restrict the rows first with employees who have more than $40,000, then do the grouping just like the first query? what is different here?

Answer 1

Sub-Query, the basic:
First, let make this query a bit easier to read :
SELECT
dname,
COUNT(*)
FROM
Department,
Employee
WHERE
dnumber = dno
AND salary > 40000
AND dno IN (
SELECT dno
FROM Employee
GROUP BY dno
HAVING COUNT(*) > 5
)
GROUP BY dname
As you can see, there is what we call a "sub-query": a query inside the query.
This is the part in dno IN (/*HERE is the Sub-query*/).
As in mathematics parenthesis are run first, so SQL will go find DNO that have more than 5 employees, producing the following query :
SELECT
dname,
COUNT(*)
FROM
Department,
Employee
WHERE
dnumber = dno
AND salary > 40000
AND dno IN (
'dno10emp', 'dno24emp', 'dno45emp'
)
GROUP BY dname
Now, you find yourself with a simple query that will produce the result:
of department that have a least one employee with >40k$ salary
and are part of the department with more the 5 employee
What's wrong ?!
Well, I'll said your "good query" isn't that good, and that's why you're struggling: It'll not bring department if they don't have at least one employee with > 40k$.
Here is the query that'll do this :
SELECT
Department.dname,
COUNT(Employee.salary)
FROM
Department
LEFT JOIN Employee
ON Department.dnumber = Employee.dno
AND Employee.salary > 40000
WHERE
Department.dnumber IN (
SELECT Employee.dno
FROM Employee
GROUP BY Employee.dno
HAVING COUNT(*) > 5
)
GROUP BY Department.dname
This will bring you all department that have at least 6 employee, then count the number of employee with at least 40K$ (a department could have 0).
Could you show me ?
As an image worth a thousand word :
SQL Fiddle
MySQL 5.6 Schema Setup:
| dname | nb | salary |
|-------------------|----|--------|
| accounting | 2 | 30000 |
| accounting | 4 | 50000 |
| boss | 6 | 150000 |
| garbage-collector | 6 | 15000 |
Query 1:
SELECT
dname,
COUNT(*)
FROM
Department,
Employee
WHERE
dnumber = dno
AND salary > 40000
GROUP BY dname
HAVING COUNT(*) > 5
Results:
| dname | COUNT(*) |
|-------|----------|
| boss | 6 |
Query 2:
SELECT
dname,
COUNT(*)
FROM
Department,
Employee
WHERE
dnumber = dno
AND salary > 40000
AND
dno IN (
SELECT dno FROM Employee
GROUP BY dno
HAVING COUNT(*) > 5
)
GROUP BY dname
Results:
| dname | COUNT(*) |
|------------|----------|
| accounting | 4 |
| boss | 6 |
Query 3:
SELECT
Department.dname,
COUNT(Employee.salary)
FROM
Department
LEFT JOIN Employee
ON Department.dnumber = Employee.dno
AND Employee.salary > 40000
WHERE
Department.dnumber IN (
SELECT Employee.dno
FROM Employee
GROUP BY Employee.dno
HAVING COUNT(*) > 5
)
GROUP BY Department.dname
Results:
| dname | COUNT(Employee.salary) |
|-------------------|------------------------|
| accounting | 4 |
| boss | 6 |
| garbage-collector | 0 |

Answer 2

See sample data below.
http://sqlfiddle.com/#!9/357d29/2
The first query will only get departments with 6 or more highy paid employees WHILE the 2nd query will get highly paid employees of those departments with 6 or more employees. Below sample will not show in the 1st query but will show in the 2nd query.
Department Employee Salary
accounting john doe 50k
jan smith 55k
dan brown 60k
eric murphy 60k
al daniels 70k
ellen boyle 30k
1st query: nothing because only five emp have > 40k salary
2nd query: All except ellen boyle. Department has > 5 employees and all except 1 has > 40k salary

Answer 3

For the record, you already got correct answers. I'll just try to explain it in a different way.
Your first query has 1 select statement. It only returns employees with salary > 40k and from departments > 5 employees. Every record will only contain information about an employee with salary > 40k and from departments > 5 employees.
Your second query has 2 select statements:
This is the first one:
Select dname, count(*)
from Department, Employee
where dnumber = dno
and salary > 40000
it returns the count of all employees, by department name who earn > 40000. There are no conditions on the count(*) here. And the condition on the salary has no power over the second select statement:
SELECT Employee.dno
FROM Employee
GROUP BY Employee.dno
HAVING COUNT(*) > 5
This one returns ALL employees in all departments. This is where we have the condition on the count(*) - but it is only applied locally, to limit the number of employees per department.
And then two statements are joined together - so, first we limit the departments to the ones we are interested in, and then from those only select high-salary employees.

Answer 4

First, never use commas in the FROM clause. Always use proper, explicit JOIN syntax.
I think the best and simplest solution uses conditional aggregation:
SELECT d.dname, SUM(CASE WHEN e.salary > 40000 THEN 1 ELSE 0 END) as num_40kplus
FROM Department d JOIN
Employee e
ON d.dno = e.dnumber
GROUP BY dname
HAVING COUNT(*) > 5;
I see no reason why a subquery would be necessary or desirable.