Difference Between Select and SelectMany

Question

I\'ve been searching the difference between Select and SelectMany but I haven\'t been able to find a suitable answer. I need to learn the differenc

Answer 1

Some SelectMany may not be necessary. Below 2 queries give the same result.

Customers.Where(c=>c.Name=="Tom").SelectMany(c=>c.Orders)

Orders.Where(o=>o.Customer.Name=="Tom")

For 1-to-Many relationship,

1. if Start from "1", SelectMany is needed, it flattens the many.
2. if Start from "Many", SelectMany is not needed. (still be able to filter from "1", also this is simpler than below standard join query)

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from o in Orders
join c in Customers on o.CustomerID equals c.ID
where c.Name == "Tom"
select o