Need some assistance with 2 SQL queries [closed] - sql

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I need some help with my exam, there are 2 questions but I don't know how to join them and which SQL statements I need.
Write a SQL statement which displays the employee id, last name,and salary of any employee whose salary is above the price of the most expensive product.
Write a SQL statement which displays the customer id of customers who purchased the product with product_id 1 but so far never purchased the product with product_id 3.
It would be nice if somebody explain me the solution.

Answer 1

With the understanding this is a practice exam, and that you don't have access to the tables/values....
You really should show what you've tried... but since you're new...
Question 1:
Return employeeID, last name and salary of all employees who have a salary greater than the price of the most expensive product (like how they avoided using the word max there...)
So this uses a subquery to get the max price from products and simply compares salary against it.
SELECT Employee_ID, Last_Name, Salary
FROM employees
WHERE salary > (Select max(price) from Products);
Question 2:
Generate two sets one for product_ID of 1 one for product_ID of 3 and make sure those with 1 are not in the group with 3.
and this uses a correlated subquery (notice how the PUR alias is in the subquery? that's why it's correlated)
So the first query gets all the customers buying product 1. The 2nd query finds all the customers buying product 3. and since we want only those customers who bought 1 not 3, we correlate the queries using not exists and we have our answer.
Exists - usually fastest as it can early exit the subquery once a single record for a customer is found.
SELECT Distinct PUR.Customer_ID
FROM purchases PUR
WHERE PRODUCT_ID = 1
and not exists (SELECT 1
FROM Purchases PUR2
WHERE Product_ID = 3
and PUR.Customer_ID = PUR2.Customer_I
There are 2-3 ways of doing this. I find the above the most efficient in most cases. However you could also do it by... a self join; or using an not IN statement
Self LEFT join (works great if you need data from multiple tables
We self join on purchases but filter each table instance to be for a specific product. we use left join as we want all records with product Id 1 and where a match is found in p2, we want to EXCLUDE those records so if the customer_ID is null that means they have no product 3 purchases
SELECT Distinct P1.customer_ID
FROM Purchases P1
LEFT JOIN Purchase P2
on P1.customer_Id = P2.customer_ID
and P2.Product_ID = 3
WHERE P1.Product_ID = 1
and P2.customer_ID is null
not in (usually slowest but works fine if subquery is for a VERY Small datasets)
SELECT Distinct PUR.Customer_ID
FROM purchases PUR
WHERE PRODUCT_ID = 1
and customer_ID not in (SELECT Customer_Id
FROM Purchases
WHERE Product_ID = 3)
Notice that neither of the 1st two answers uses a join; though a correlated subquery is close.