**Describe the bug**
If you make a query to MySQL Engine Table with joined MergeTree Engine Table, you use any columns of the latter.


**How to reproduce**
* ClickHouse server 20.8.2
* ClickHouse client version 20.8.2.3 (official build).

```sql
CREATE TABLE mysql_table
(
    `id` UInt64,
    `name` Nullable(String),
)
ENGINE = MySQL('test:3306', 'test', 'test', 'test', 'test');
```

```sql
CREATE TABLE ch_table
(
    `id` UInt64,
    `mysql_item_id` UInt64,
    `d` DateTime
)
ENGINE = MergeTree()
PARTITION BY toYYYYMM(d)
ORDER BY d;
```

Query 
```sql
    SELECT *
    FROM mysql_table AS t_mysql_table
    LEFT JOIN ch_table AS t_ch_table 
         ON t_ch_table.mysql_item_id = t_mysql_table.id
    WHERE t_ch_table.id > 100
```

**Expected behavior**
Returns empty result without errors.

**Error message and/or stacktrace**
```
Code: 47. DB::Exception: Received from test:9000. DB::Exception: Missing columns: 't_ch_table.id' while processing query: 't_ch_table.id > 100', required columns: 't_ch_table.id', source columns: 'id', 'name'. 
```

{
        static State state;
        return state;
    }
private:
    explicit State()
        : context(getContext().context)
    {
        tryRegisterFunctions();
        DatabasePtr database = std::make_shared<DatabaseMemory>("test", context);
        database->attachTable("table", StorageMemory::create(StorageID("test", "table"), ColumnsDescription{columns}, ConstraintsDescription{}));
        DatabaseCatalog::instance().attachDatabase("test", database);
        context.setCurrentDatabase("test");
    }
};
static void check(const std::string & query, const std::string & expected, const Context & context, const NamesAndTypesList & columns)
{
    ParserSelectQuery parser;
    ASTPtr ast = parseQuery(parser, query, 1000, 1000);
    SelectQueryInfo query_info;
    query_info.syntax_analyzer_result = TreeRewriter(context).analyzeSelect(ast, columns);
    query_info.query = ast;
    std::string transformed_query = transformQueryForExternalDatabase(query_info, columns, IdentifierQuotingStyle::DoubleQuotes, "test", "table", context);
    EXPECT_EQ(transformed_query, expected);
}
TEST(TransformQueryForExternalDatabase, InWithSingleElement)