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When you have a 'List of Lists', Select will give you a list containing lists. SelectMany flattens them into a single, unified stream.
Imagine a Department has many Employees. If you want a list of ALL employees across all departments, SelectMany is your best friend.
// Using Select (returns List<List<Employee>>)
var nested = departments.Select(d => d.Employees);
// Using SelectMany (returns List<Employee>)
var flattened = departments.SelectMany(d => d.Employees);
You can use SelectMany to combine every item in List A with every item in List B. This is the LINQ equivalent of a SQL **CROSS JOIN**.
Q: "When is SelectMany too expensive?"
Architect Answer: "In EF Core, SelectMany is translated into a **JOIN**. While powerful, be careful of 'Cartesian Explosion' where the resulting result set becomes massive. Always combine SelectMany with a Where clause to keep the result set manageable for your server's memory."
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On this page
1. One-to-Many Relationships 2. Cross Joins (Cross Products) 3. Architect Insight