SQL.How do GROUP BY and DISTINCT treat NULL values, even though NULL = NULL evaluates to UNKNOWN?

`GROUP BY` and `DISTINCT` treat NULL values as **not distinct from each other**, even though `NULL = NULL` evaluates to `UNKNOWN`.

- **GROUP BY:** All NULL values in a grouping column form one group.
- **DISTINCT:** Multiple NULL values in a selected column become one NULL.

For values `10, 10, NULL, NULL`:

```sql
SELECT value, COUNT(*)
FROM example
GROUP BY value;
-- 10   → 2
-- NULL → 2

SELECT DISTINCT value FROM example;
-- 10
-- NULL
```

With multiple columns, grouping and deduplication consider the **entire combination of values**.

**Interview answer:** “GROUP BY and DISTINCT use ‘not distinct’ semantics rather than ordinary equality. This allows NULL values to belong to the same group or be treated as duplicates.”