pub struct NamedParenthesizedList {
    pub key: Ident,
    pub name: Option<Ident>,
    pub values: Vec<Ident>,
}

UNION = (tbl_name[,tbl_name]...)
ENGINE = ReplicatedMergeTree('/table_name','{replica}', ver)
ENGINE = SummingMergeTree([columns])

The option key (identifier) for this named list.

Optional secondary name associated with the key.

The list of identifier values for the key.