sql_parser_query.rb

Query = Struct.new(:attributes, :source, :conditions, :order_by)
class SQLParser
  attr_reader :query
  def initialize(string)
    @tokens = string.scan(/\(|\)|:?\w+|[\+\-\/\*=]+|[<>]=?/)
    @query = Query.new
  end

  def next_token
    @tokens.shift
  end

  def parse
    case token = next_token
    when 'select'
      @query.attributes = parse_until('from')
      parse
    when 'from'
      @query.source = parse_until('where')
      parse
    when 'where'
      @query.conditions = parse_until('order')
      parse
    when 'order'
      @query.order_by = []
      @query.order_by << parse until @tokens.empty?
    when /\d+\.\d+/ then token.to_f
    when /\d+/ then token.to_i
    when /[<>]=?/ then token.to_sym
    else token.to_sym
    end
  end

  def parse_until(token_close)
    list = []
    list << parse until @tokens.empty? || @tokens.first == token_close
    list
  end
end

sql = SQLParser.new('select id, name from table where age > 30 order age desc')
sql.parse
pp sql.query # <struct Query attributes=[:id, :name], source=[:table], conditions=[:age, :>, 30], order_by=[:age, :desc]>