DOCAPI-6183: JOIN syntax limitations. (#5424)

docs/en/query_language/dicts/external_dicts_dict_sources.md

@@ -54,7 +54,7 @@ Setting fields:
 
 ## Executable File {#dicts-external_dicts_dict_sources-executable}
 
-Working with executable files depends on [how the dictionary is stored in memory](external_dicts_dict_layout.md). If the dictionary is stored using `cache` and `complex_key_cache`, ClickHouse requests the necessary keys by sending a request to the executable file's `STDIN`.
+Working with executable files depends on [how the dictionary is stored in memory](external_dicts_dict_layout.md). If the dictionary is stored using `cache` and `complex_key_cache`, ClickHouse requests the necessary keys by sending a request to the executable file's STDIN. Otherwise, ClickHouse starts executable file and treats its output as dictionary data.
 
 Example of settings:
 

docs/en/query_language/select.md

@@ -450,7 +450,7 @@ See the standard [SQL JOIN](https://en.wikipedia.org/wiki/Join_(SQL)) descriptio
 
 #### Multiple JOIN
 
-Performing queries, ClickHouse rewrites multiple joins into the combination of two-table joins and processes them sequentially. If there are four tables for join ClickHouse joins the first and the second, then joins the result with the third table, and at the last step, it joins the fourth one.
+Performing queries, ClickHouse rewrites multiple joins into the sequence of two-table joins. If there are four tables for join ClickHouse joins the first and the second, then joins the result with the third table, and at the last step, it joins the fourth one.
 
 If a query contains `WHERE` clause, ClickHouse tries to push down filters from this clause into the intermediate join. If it cannot apply the filter to each intermediate join, ClickHouse applies the filters after all joins are completed.
 
@@ -531,7 +531,7 @@ LIMIT 10
 ```
 
 Subqueries don't allow you to set names or use them for referencing a column from a specific subquery.
-The columns specified in `USING` must have the same names in both subqueries, and the other columns must be named differently. You can use aliases to change the names of columns in subqueries (the example uses the aliases 'hits' and 'visits').
+The columns specified in `USING` must have the same names in both subqueries, and the other columns must be named differently. You can use aliases to change the names of columns in subqueries (the example uses the aliases `hits` and `visits`).
 
 The `USING` clause specifies one or more columns to join, which establishes the equality of these columns. The list of columns is set without brackets. More complex join conditions are not supported.
 
@@ -550,6 +550,16 @@ While joining tables, the empty cells may appear. The setting [join_use_nulls](.
 
 If the `JOIN` keys are [Nullable](../data_types/nullable.md) fields, the rows where at least one of the keys has the value [NULL](syntax.md#null-literal) are not joined.
 
+#### Syntax Limitations
+
+For multiple `JOIN` clauses in the single `SELECT` query:
+
+- Taking all the columns via `*` is available only if tables are joined, not subqueries.
+- The `PREWHERE` clause is not available.
+
+For `ON`, `WHERE` and `GROUP BY` clauses:
+
+- Arbitrary expressions cannot be used in `ON`, `WHERE` and `GROUP BY` clauses, but you can define an expression in `SELECT` clause and then use it via alias in these clauses.
 
 ### WHERE Clause