clustered vs. non-clustered indexes.

**Clustered Index:**

- **Definition:** Determines the physical order of data rows in the table.

- **Characteristics:** Only one clustered index per table. The table is sorted and stored on disk according to this index.

- **Use Case:** Efficient for range queries and sorting operations.

**Non-Clustered Index:**

- **Definition:** Creates a separate structure from the data rows, with pointers to the location of the data.

- **Characteristics:** Multiple non-clustered indexes can exist per table. Does not affect the physical order of the table.

- **Use Case:** Useful for optimizing queries on non-primary key columns.