# NULL
__*Definition of NULL*__
__*NULL Behavior*__
+ **`NULL`** is an `SQL` keyword that defines the data types that allow missing values
  + `Unknown`: A value exists, but the exact value is not yet known
  + `N/A`: If a value is specified, the value will not be accurately representative
  + Null values are allowed by default
  + If the column does not allow null values, use the **`NOT NULL`** declaration to ensure data integrity
  + **`NULL`** is the minimum value in **`ORDER BY`**
  + For example:
```
tidb> SELECT NULL = NULL, NULL != NULL, NULL <=> NULL, NULL <=> 'X';
+-------------+--------------+---------------+--------------+
| NULL = NULL | NULL != NULL | NULL <=> NULL | NULL <=> 'X' |
+-------------+--------------+---------------+--------------+
|        NULL |         NULL |             1 |            0 |
+-------------+--------------+---------------+--------------+
1 row in set (0.00 sec)
```

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