Undo renaming and make review changes

docs/en/sql-reference/aggregate-functions/index.md

@@ -60,7 +60,7 @@ SELECT groupArray(y) FROM t_null_big
 
 `groupArray` does not include `NULL` in the resulting array.
 
-You can use [COALESCE](../../sql-reference/functions/null-functions.md#coalesce) to change NULL into a value that makes sense in your use case. For example: `avg(COALESCE(column, 0))` with use the column value in the aggregation or zero if NULL:
+You can use [COALESCE](../../sql-reference/functions/functions-for-nulls.md#coalesce) to change NULL into a value that makes sense in your use case. For example: `avg(COALESCE(column, 0))` with use the column value in the aggregation or zero if NULL:
 
 ``` sql
 SELECT

docs/en/sql-reference/functions/functions-for-nulls.md

@@ -1,5 +1,5 @@
 ---
-slug: /en/sql-reference/functions/null-functions
+slug: /en/sql-reference/functions/functions-for-nulls
 sidebar_position: 135
 sidebar_label: Nullable
 ---
@@ -56,7 +56,7 @@ Result:
 
 ## isNullable
 
-Returns `1` if a column allows null values, `0` otherwise.
+Returns `1` if a column is [Nullable](../data-types/nullable.md) (i.e allows `NULL` values), `0` otherwise.
 
 **Syntax**
 
@@ -78,18 +78,18 @@ isNullable(x)
 Query:
 
 ``` sql
-CREATE TABLE example_table (nullable_column Nullable(UInt32), ordinary_column UInt32) ENGINE = Log;
-INSERT INTO example_table (nullable_column, ordinary_column) VALUES (1,1), (2, 2), (3,3);
-SELECT isNullable(nullable_column), isNullable(ordinary_column) FROM example_table;    
+CREATE TABLE tab (ordinary_col UInt32, nullable_col Nullable(UInt32)) ENGINE = Log;
+INSERT INTO tab (ordinary_col, nullable_col) VALUES (1,1), (2, 2), (3,3);
+SELECT isNullable(ordinary_col), isNullable(nullable_col) FROM tab;    
 ```
 
 Result:
 
 ``` text
-   ┌─isNullable(nullable_column)─┬─isNullable(ordinary_column)─┐
-1. │                           1 │                           0 │
-2. │                           1 │                           0 │
-3. │                           1 │                           0 │
+   ┌───isNullable(ordinary_col)──┬───isNullable(nullable_col)──┐
+1. │                           0 │                           1 │
+2. │                           0 │                           1 │
+3. │                           0 │                           1 │
    └─────────────────────────────┴─────────────────────────────┘
 ```
 
@@ -144,7 +144,7 @@ This function will consider two `NULL` values as identical and will return `true
 equals behavior where comparing two `NULL` values would return `NULL`.
 
 :::note
-This function can only be used as part of a JOIN ON section of a query. The function is unimplemented for any other use.
+This function is an internal function used by the implementation of JOIN ON. Please do not use it manually in queries.
 :::
 
 **Syntax**

docs/en/sql-reference/statements/select/where.md

@@ -15,7 +15,7 @@ If there is a `WHERE` clause, it must contain an expression with the `UInt8` typ
 There is a filtering optimization called [PREWHERE](../../../sql-reference/statements/select/prewhere.md).
 :::
 
-If you need to test a value for [NULL](../../../sql-reference/syntax.md#null-literal), use [IS NULL](../../operators/index.md#operator-is-null) and [IS NOT NULL](../../operators/index.md#is-not-null) operators or [isNull](../../../sql-reference/functions/null-functions.md#isnull) and [isNotNull](../../../sql-reference/functions/null-functions.md#isnotnull) functions.
+If you need to test a value for [NULL](../../../sql-reference/syntax.md#null-literal), use [IS NULL](../../operators/index.md#operator-is-null) and [IS NOT NULL](../../operators/index.md#is-not-null) operators or [isNull](../../../sql-reference/functions/functions-for-nulls.md#isnull) and [isNotNull](../../../sql-reference/functions/functions-for-nulls.md#isnotnull) functions.
 Otherwise an expression with `NULL` never passes.
 
 **Example**

docs/ru/sql-reference/functions/null-functions.md

@@ -212,7 +212,7 @@ SELECT nullIf(1, 2);
 
 ## assumeNotNull {#assumenotnull}
 
-Приводит значение типа [Nullable](../../sql-reference/functions/null-functions.md) к не `Nullable`, если значение не `NULL`.
+Приводит значение типа [Nullable](../../sql-reference/functions/functions-for-nulls.md) к не `Nullable`, если значение не `NULL`.
 
 ``` sql
 assumeNotNull(x)

docs/ru/sql-reference/statements/select/where.md

@@ -14,7 +14,7 @@ ClickHouse использует в выражении индексы, если
 :::note Примечание
 Существует оптимизация фильтрации под названием [PREWHERE](prewhere.md).
 :::
-Если в секции необходимо проверить [NULL](../../../sql-reference/syntax.md#null-literal), то используйте операторы [IS NULL](../../operators/index.md#operator-is-null) и [IS NOT NULL](../../operators/index.md#is-not-null), а также соответствующие функции [isNull](../../../sql-reference/functions/null-functions.md#isnull) и [isNotNull](../../../sql-reference/functions/null-functions.md#isnotnull). В противном случае выражение будет считаться всегда не выполненным.
+Если в секции необходимо проверить [NULL](../../../sql-reference/syntax.md#null-literal), то используйте операторы [IS NULL](../../operators/index.md#operator-is-null) и [IS NOT NULL](../../operators/index.md#is-not-null), а также соответствующие функции [isNull](../../../sql-reference/functions/functions-for-nulls.md#isnull) и [isNotNull](../../../sql-reference/functions/functions-for-nulls.md#isnotnull). В противном случае выражение будет считаться всегда не выполненным.
 
 **Пример**
 

docs/zh/sql-reference/functions/null-functions.md

@@ -181,7 +181,7 @@ SELECT coalesce(mail, phone, CAST(icq,'Nullable(String)')) FROM aBook
 
 ## assumeNotNull {#assumenotnull}
 
-将[可为空](../../sql-reference/functions/null-functions.md)类型的值转换为非`Nullable`类型的值。
+将[可为空](../../sql-reference/functions/functions-for-nulls.md)类型的值转换为非`Nullable`类型的值。
 
     assumeNotNull(x)
 

docs/zh/sql-reference/statements/select/where.md

@@ -15,7 +15,7 @@ sidebar_label: WHERE
 有一个叫做过滤优化 [prewhere](../../../sql-reference/statements/select/prewhere.md) 的东西.
 :::
 
-如果需要测试一个 [NULL](../../../sql-reference/syntax.md#null-literal) 值，请使用 [IS NULL](../../operators/index.md#operator-is-null) and [IS NOT NULL](../../operators/index.md#is-not-null) 运算符或 [isNull](../../../sql-reference/functions/null-functions.md#isnull) 和 [isNotNull](../../../sql-reference/functions/null-functions.md#isnotnull) 函数。否则带有 NULL 的表达式永远不会通过。
+如果需要测试一个 [NULL](../../../sql-reference/syntax.md#null-literal) 值，请使用 [IS NULL](../../operators/index.md#operator-is-null) and [IS NOT NULL](../../operators/index.md#is-not-null) 运算符或 [isNull](../../../sql-reference/functions/functions-for-nulls.md#isnull) 和 [isNotNull](../../../sql-reference/functions/functions-for-nulls.md#isnotnull) 函数。否则带有 NULL 的表达式永远不会通过。
 
 **示例**