Bits of .NET

-- ❌ SLOW (subquery runs per row)
SELECT 
  c.Name,
  (SELECT TOP 1 OrderDate 
   FROM Orders 
   WHERE CustomerId = c.CustomerId 
   ORDER BY OrderDate DESC) AS LastOrder
FROM Customers c;

-- ✅ FAST (CROSS APPLY optimized)
SELECT c.Name, o.OrderDate
FROM Customers c
CROSS APPLY (
  SELECT TOP 1 OrderDate
  FROM Orders
  WHERE CustomerId = c.CustomerId
  ORDER BY OrderDate DESC
) o;

Result: 10-100x faster on large tables.

When to Use: Need top N related rows, complex subquery logic.