I use an expression as `update_field` because this feature is not implemented yet: https://github.com/ClickHouse/ClickHouse/issues/6084

20.5 generates an incorrect query in this case:
```WHERE `date_add(updated_date, interval 15 minute)` >= '2020-07-02 16:40:06'```

The exception:
```mysqlxx::BadQuery: Unknown column 'date_add(updated_date, interval 15 minute)' in 'where clause'```

The problem is here: 
https://github.com/ClickHouse/ClickHouse/blob/b81c66a03148afc02647df3c11aec40382329912/src/Dictionaries/ExternalQueryBuilder.cpp#L172
The change was introduced in this PR: https://github.com/ClickHouse/ClickHouse/pull/11079

#include "ExternalQueryBuilder.h"
#include <IO/WriteBuffer.h>
#include <IO/WriteBufferFromString.h>
#include <IO/WriteHelpers.h>
#include <boost/range/join.hpp>
#include <ext/range.h>
#include "DictionaryStructure.h"
#include "writeParenthesisedString.h"
namespace DB
{
namespace ErrorCodes
{
    extern const int UNSUPPORTED_METHOD;
}
ExternalQueryBuilder::ExternalQueryBuilder(
    const DictionaryStructure & dict_struct_,
    const std::string & db_,
    const std::string & schema_,
    const std::string & table_,
    const std::string & where_,
    IdentifierQuotingStyle quoting_style_)
    : dict_struct(dict_struct_), db(db_), schema(schema_), table(table_), where(where_), quoting_style(quoting_style_)
{}
void ExternalQueryBuilder::writeQuoted(const std::string & s, WriteBuffer & out) const