code: function columnsToSQL(columns) {
      return columns
          .map(column => {
              let str = exprToSQL(column.expr);
  
              if (column.as !== null) {
                  str += ' AS ';
                  if (column.as.match(/^[a-z_][0-9a-z_]*$/i)) str += identifierToSql(column.as);
                  else str += '"' + column.as + '"';
              }
  
              return str;
          })
          .join(', ');
  }
docs: Stringify column expressions
  
  @param {Array} columns
  @return {string}