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A non-clustered index is a **separate structure** stored **outside the main table** that contains:

1. **sorted keys** (the ones the index is built on),
2. **pointers (references)** to the actual rows in the table.

### The principle

- The data in the table keeps its own order,
- A non-clustered index creates a "pointer card", like a separate table of contents that knows where every row sits.

### Example

```sql
CREATE NONCLUSTERED INDEX idx_users_email
ON users(email);
```

Now the database builds a structure with sorted `email` values and references to the real rows.
When this query runs:

```sql
SELECT * FROM users WHERE email = 'a@mail.com';
```

SQL first looks up `email` in the index,
then follows the reference to the row in the table (this is called a **lookup**).

### Details

- You can create **many non-clustered indexes** on one table.
- They speed up selection (`SELECT`, `WHERE`, `JOIN`, `ORDER BY`).
- But inserting/updating a row means **updating every index** that the changed field is part of.

### Difference from a clustered index

- A **clustered** index sets the physical order of rows in the table.
- A **non-clustered** index is a "pointer" to the data, stored separately.

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