Basing on the picture I need to write the following query:
- returning all employees older than 50 years and
- participated in development of more than 10 products
The resultset should contain the following columns:
- Name of the emloyee
- Employee's age
- The last product's date (basing on ProductEmployee.Date)
My solution is:
select e0.Name, e0.Age, tt.Name as TheLastProduct
from Employee e0
join (
select e.ID
from Employee e
join ProductEmployee pe on pe.EmployeeID = e.ID
join Product p on p.ID = pe.ProductID
where e.Age > 50
group by e.ID
having COUNT( p.ID) > 10
) e1 on e1.ID = e0.ID
cross apply
(
select top 1 p1.ID, p1.Name from Product p1
join ProductEmployee pe2 on p1.ID = pe2.ProductID
where pe2.EmployeeID = e1.ID order by pe2.Date desc
) tt
but I was told there might be a better solution. I wonder what the better solution is.
I would have done
SELECT ...
FROM Employee AS E
WHERE e.Age > 50
AND
(
SELECT COUNT(*)
FROM ProductEmployee pe
on pe.EmployeeID = e.ID
join Product p
on p.ID = pe.ProductID
) > 10
CROSS APPLY ...
Difference between this, and yours, is that I only reference the Employee table once, so will probably be more efficient that yours (which re-references the Employee in the sub-query). It is also possible that mine will only COUNT the Products for rows where Employee > 50 - which would improve performance (assuming that there are some young people in the company!!)
If you have studied ROW_NUMBER ... OVER then you could use that to BOTH find the most-recent-product and also the COUNT of matches, in the same sub-query, which would be more efficient.
1 Like
I'd write the query the most natural way, and let SQL's optimizer decide the details of how to execute it. A correlated subquery can sometimes be inefficient. Also, you need to return the last product date, which a correlated subquery in a WHERE won't allow you to do.
SELECT e.Name, e.Age, p.Last_Product_Date
FROM Employee e
INNER JOIN (
SELECT EmployeeID, MAX(Date) AS Last_Product_Date
FROM Product
GROUP BY EmployeeID
HAVING COUNT(*) > 10
) AS p ON p.EmployeeID = e.ID
WHERE e.Age > 50