What is CROSS APPLY vs OUTER APPLY?

Short answer: APPLY invokes a table-valued expression per outer row. CROSS APPLY is like INNER JOIN (outer row must produce rows). OUTER APPLY is like LEFT JOIN (keeps outer rows with NULLs). Useful for TOP N per group and TVFs.

Sample solution

-- Top 2 orders per customer
SELECT c.CustomerId, c.Name, o.OrderId, o.Amount
FROM Customers c
CROSS APPLY (
    SELECT TOP (2) OrderId, Amount
    FROM Orders o
    WHERE o.CustomerId = c.CustomerId
    ORDER BY Amount DESC
) o;

TOP N per group via CROSS APPLY is a classic SQL Server interview flex.