SQL.How can you compare two nullable values safely, and how does IS DISTINCT FROM differ from <>?

`<>` returns **UNKNOWN** if either value is `NULL`.

`IS DISTINCT FROM` provides a NULL-safe comparison: it always returns **TRUE or FALSE**, treating two NULLs as equal and one NULL as different from a non-NULL value.

| Comparison | `a <> b` | `a IS DISTINCT FROM b` |
|---|---|---|
| `5`, `5` | FALSE | FALSE |
| `5`, `10` | TRUE | TRUE |
| `5`, `NULL` | UNKNOWN | TRUE |
| `NULL`, `NULL` | UNKNOWN | FALSE |

```sql
WHERE old_value IS DISTINCT FROM new_value
-- Detects changes, including transitions to or from NULL
```

For NULL-safe equality, use the opposite:

```sql
WHERE a IS NOT DISTINCT FROM b
-- TRUE when values are equal or both NULL
```

**Interview answer:** “Unlike <>, IS DISTINCT FROM always returns TRUE or FALSE, even with NULLs. It treats two NULLs as equal, making it useful for reliably detecting changes between nullable values.”