build-sql.rb

require 'erb'

sql_template = <<END
SELECT
FROM users
WHERE true
% if gender
  AND gender = :gender
% end
% if age_min
  AND age   >= :age_min
% end
% if age_max
  AND age   <= :age_max
% end
ORDER BY id
END

erb = ERB.new(sql_template, nil, '%')
#puts erb.src

### No condition
gender = age_min = age_max = nil
puts erb.result(binding())
# -- result --
#   SELECT
#   FROM users
#   WHERE true
#   ORDER BY id

### Gender specified
gender = 'F'
puts erb.result(binding())
# -- result --
#   SELECT
#   FROM users
#   WHERE true
#     AND gender = :gender
#   ORDER BY id

### All specified
gender = 'F'
age_min = 20
age_max = 30
puts erb.result(binding())
# -- result --
#   SELECT
#   FROM users
#   WHERE true
#     AND gender = :gender
#     AND age   >= :age_min
#     AND age   <= :age_max
#   ORDER BY id