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`NULLS FIRST` and `NULLS LAST` are parameters that control **where** `NULL` values end up when sorting in SQL.

By default `NULL` is treated as an *undefined* value, and different DBMSes (PostgreSQL, Oracle, SQLite, and others) place it differently.
To set the order explicitly, use these parameters.

### `NULLS FIRST`

Places all `NULL`s **at the start** of the result.

```sql
SELECT name, salary
FROM employees
ORDER BY salary ASC NULLS FIRST;
```

Even with ascending order, rows with `NULL` come **first**.

### `NULLS LAST`

Places all `NULL`s **at the end** of the result.

```sql
SELECT name, salary
FROM employees
ORDER BY salary DESC NULLS LAST;
```

Regardless of the descending order, empty values end up **at the very bottom** of the list.

### Why this matters

Without `NULLS FIRST/LAST`, the order of rows with `NULL` can differ:

- in some systems they're treated as "less than everything",
- in others, as "greater than everything".

So these parameters exist to let you **control the position of empty values explicitly**.

**Summary:**

- `NULLS FIRST`: `NULL` at the start of the result;
- `NULLS LAST`: `NULL` at the end;
- used with `ORDER BY` to set an exact sort order when there are gaps in the data.

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