How to make this complex query more efficient? - sql

Question

I want to select employees, having more than 10 products and older than 50. I also want to have their last product selected. I use the following query:
SELECT
PE.EmployeeID, E.Name, E.Age,
COUNT(*) as ProductCount,
(SELECT TOP(1) xP.Name
FROM ProductEmployee xPE
INNER JOIN Product xP ON xPE.ProductID = xP.ID
WHERE xPE.EmployeeID = PE.EmployeeID
AND xPE.Date = MAX(PE.Date)) as LastProductName
FROM
ProductEmployee PE
INNER JOIN
Employee E ON PE.EmployeeID = E.ID
WHERE
E.Age > 50
GROUP BY
PE.EmployeeID, E.Name, E.Age
HAVING
COUNT(*) > 10
Here is the execution plan link: https://www.dropbox.com/s/rlp3bx10ty3c1mf/ximExPlan.sqlplan?dl=0
However it takes too much time to execute it. What's wrong with it? Is it possible to make a more efficient query?
I have one limitation - I can not use CTE. I believe it will not bring performance here anyway though.

Answer 1

Before creating Index I believe we can restructure the query.
Your query can be rewritten like this
SELECT E.ID,
E.NAME,
E.Age,
CS.ProductCount,
CS.LastProductName
FROM Employee E
CROSS apply(SELECT TOP 1 P.NAME AS LastProductName,
ProductCount
FROM (SELECT *,
Count(1)OVER(partition BY EmployeeID) AS ProductCount -- to find product count for each employee
FROM ProductEmployee PE
WHERE PE.EmployeeID = E.Id) PE
JOIN Product P
ON PE.ProductID = P.ID
WHERE ProductCount > 10 -- to filter the employees who is having more than 10 products
ORDER BY date DESC) CS -- To find the latest sold product
WHERE age > 50

Answer 2

This should work:
SELECT *
FROM Employee AS E
INNER JOIN (
SELECT PE.EmployeeID
FROM ProductEmployee AS PE
GROUP BY PE.EmployeeID
HAVING COUNT(*) > 10
) AS PE
ON PE.EmployeeID = E.ID
CROSS APPLY (
SELECT TOP (1) P.*
FROM Product AS P
INNER JOIN ProductEmployee AS PE2
ON PE2.ProductID = P.ID
WHERE PE2.EmployeeID = E.ID
ORDER BY PE2.Date DESC
) AS P
WHERE E.Age > 50;
Proper indexes should speed query up.
You're filtering by Age, so followining one should help:
CREATE INDEX ix_Person_Age_Name
ON Person (Age, Name);
Subquery that finds emploees with more than 10 records should be calculated first and CROSS APPLY should bring back data more efficient with TOP operator rather than comparing it to MAX value.
Answer by #Prdp is great, but I thought I'll drop an alternative in. Sometimes windowed functions do not work very well and it's worth to replace them with ol'good subqueries.
Also, do not use datetime, use datetime2. This is suggest by Microsoft:
https://msdn.microsoft.com/en-us/library/ms187819.aspx
Use the time, date, datetime2 and datetimeoffset data
types for new work. These types align with the SQL Standard. They are
more portable. time, datetime2 and datetimeoffset provide
more seconds precision. datetimeoffset provides time zone support
for globally deployed applications.
By the way, here's a tip. Try to name your surrogate primary keys after table, so they become more meaningful and joins feel more natural. I.E.:
In Employee table replace ID with EmployeeID
In Product table replace ID with ProductID
I find these a good practice.

Answer 3

with usersOver50with10productsOrMore (employeeID, productID, date, id, name, age, products ) as (
select employeeID, productID, date, id, name, age, count(productID) from productEmployee
join employee on productEmployee.employeeID = employee.id
where age >= 50
group by employeeID, productID, date, id, name, age
having count(productID) >= 10
)
select sfq.name, sfq.age, pro.name, sfq.products, max(date) from usersOver50with10productsOrMore as sfq
join product pro on sfq.productID = pro.id
group by sfq.name, sfq.age, pro.name, sfq.products
;
There is no need to find the last productID for the entire table, just filler the last product from the results of employees with 10 or more products and over the age of 50.