Method parse_optional_alias

src/sql-parser/src/parser.rs:7180–7214

Parse `AS identifier` (or simply `identifier` if it's not a reserved keyword) Some examples with aliases: `SELECT 1 foo`, `SELECT COUNT(*) AS cnt`, `SELECT ... FROM t1 foo, t2 bar`, `SELECT ... FROM (...) AS bar`

(&mut self, is_reserved: F)

    /// Some examples with aliases: `SELECT 1 foo`, `SELECT COUNT(*) AS cnt`,
    /// `SELECT ... FROM t1 foo, t2 bar`, `SELECT ... FROM (...) AS bar`
    fn parse_optional_alias<F>(&mut self, is_reserved: F) -> Result<Option<Ident>, ParserError>
    where
        F: FnOnce(Keyword) -> bool,
    {
        let after_as = self.parse_keyword(AS);
        match self.next_token() {
            // Do not accept `AS OF`, which is reserved for providing timestamp information
            // to queries.
            Some(Token::Keyword(OF)) => {
                self.prev_token();
                if after_as {
                    self.prev_token();
                }
                Ok(None)
            }
            // Accept any other identifier after `AS` (though many dialects have restrictions on
            // keywords that may appear here). If there's no `AS`: don't parse keywords,
            // which may start a construct allowed in this position, to be parsed as aliases.
            // (For example, in `FROM t1 JOIN` the `JOIN` will always be parsed as a keyword,
            // not an alias.)
            Some(Token::Keyword(kw)) if after_as || !is_reserved(kw) => Ok(Some(kw.into())),
            Some(Token::Ident(id)) => Ok(Some(self.new_identifier(id)?)),
            not_an_ident => {
                if after_as {
                    return self.expected(
                        self.peek_prev_pos(),
                        "an identifier after AS",
                        not_an_ident,
                    );
                }
                self.prev_token();
                Ok(None) // no alias found
            }
        }
    }

    /// Parse `AS identifier` when the AS is describing a table-valued object,
    /// like in `... FROM generate_series(1, 10) AS t (col)`. In this case