sexp_to_sql.rb

class Synthetizer
  def initialize
    @heap = []
  end

  def push(element)
    @heap << element
  end

  def synth(sexp)
    head, *tail = *sexp
    case head
    when Symbol
      send("synth_#{head}", tail)
    when Array
      synth(head)
      synth(tail)
    end
    @heap.join(' ')
  end

  def synth_select(sexp)
    push 'SELECT'
    if sexp.empty?
      push '*'
    else
      push sexp.join(', ')
    end
  end

  def synth_from(sexp)
    return if sexp.empty?
    push 'FROM'
    push sexp.first
    synth_join(sexp.drop(1))
  end

  def synth_join(sexp)
    return if sexp.empty?
    push 'INNER JOIN'
    push sexp.first
    push 'ON'
    synth_cond(sexp.drop(1))
    synth_join(sexp.drop(2))
  end

  def synth_where(sexp)
    return if sexp.empty?
    push 'WHERE'
    synth_cond(sexp)
  end

  def synth_cond(sexp)
    push sexp and return unless sexp.is_a? Array
    return if sexp.empty?
    head, *tail = *sexp
    case head
    when :'='
      left, right = *tail
      synth_cond(left)
      push '='
      synth_cond(right)
    when :and
      left, right = *tail
      synth_cond(left)
      push 'AND'
      synth_cond(right)
    when Array
      synth_cond(head)
      synth_cond(tail)
    end
  end
end

sql1 = [[:select, 'campo1', 'campo2'], [:from, 'tabela1']]
sql2 = [[:select],
        [:from, 'tabela1',
         'tabela2', [:'=', 'id', 'tabela1_id']],
        [:where, [:and, [:'=', 'campo1', 5], [:'=', 'campo2', 'campo1']]]]
sql3 = [[:select, 'campo1'], [:from, 'tabela1',
                    'tabela2', [:'=', 'tabela1.id', 'tabela1_id'],
                    'tabela3', [:'=', 'tabela2.id', 'tabela2_id'],
                    'tabela4', [:'=', 'tabela1.id', 'tabela3_id']]]
puts Synthetizer.new.synth(sql1)
puts Synthetizer.new.synth(sql2)
puts Synthetizer.new.synth(sql3)