# 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
  + Nulls are allowed by default
  + If the column does not allow null values, use the**NOT NUL** 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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