Merge pull request #18547 from adevyatova/annadevyatova-DOCSUP-4437-nullmod

DOCSUP-4437: NULL modifiers

docs/en/operations/settings/settings.md

@@ -2470,6 +2470,17 @@ Possible values:
 
 Default value: `0`.
 
+## data_type_default_nullable {#data_type_default_nullable}
+
+Allows data types without explicit modifiers [NULL or NOT NULL](../../sql-reference/statements/create/table.md#null-modifiers) in column definition will be [Nullable](../../sql-reference/data-types/nullable.md#data_type-nullable).
+
+Possible values:
+
+- 1 — The data types in column definitions are set to `Nullable` by default.
+- 0 — The data types in column definitions are set to not `Nullable` by default.
+
+Default value: `0`.
+
 ## execute_merges_on_single_replica_time_threshold {#execute-merges-on-single-replica-time-threshold}
 
 Enables special logic to perform merges on replicas.

docs/en/sql-reference/statements/create/table.md

@@ -16,8 +16,8 @@ By default, tables are created only on the current server. Distributed DDL queri
 ``` sql
 CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
 (
-    name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1] [compression_codec] [TTL expr1],
-    name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2] [compression_codec] [TTL expr2],
+    name1 [type1] [NULL|NOT NULL] [DEFAULT|MATERIALIZED|ALIAS expr1] [compression_codec] [TTL expr1],
+    name2 [type2] [NULL|NOT NULL] [DEFAULT|MATERIALIZED|ALIAS expr2] [compression_codec] [TTL expr2],
     ...
 ) ENGINE = engine
 ```
@@ -57,6 +57,14 @@ In all cases, if `IF NOT EXISTS` is specified, the query won’t return an error
 
 There can be other clauses after the `ENGINE` clause in the query. See detailed documentation on how to create tables in the descriptions of [table engines](../../../engines/table-engines/index.md#table_engines).
 
+## NULL Or NOT NULL Modifiers {#null-modifiers}
+
+`NULL` and `NOT NULL` modifiers after data type in column definition allow or do not allow it to be [Nullable](../../../sql-reference/data-types/nullable.md#data_type-nullable). 
+
+If the type is not `Nullable` and if `NULL` is specified, it will be treated as `Nullable`; if `NOT NULL` is specified, then no. For example, `INT NULL` is the same as `Nullable(INT)`. If the type is `Nullable` and `NULL` or `NOT NULL` modifiers are specified, the exception will be thrown.
+
+See also [data_type_default_nullable](../../../operations/settings/settings.md#data_type_default_nullable) setting.
+
 ## Default Values {#create-default-values}
 
 The column description can specify an expression for a default value, in one of the following ways: `DEFAULT expr`, `MATERIALIZED expr`, `ALIAS expr`.