validates_uniqueness_of :mobile
validates_uniqueness_of :email
validates_uniqueness_of :card_num

name   mobile     email            card_num
============================================
Mary   65666678   mary@gmail.com   A001

name   mobile     email            card_num
============================================
Peter  65666678   peter@gmail.com  A002

name   mobile     email            card_num
============================================
Peter  74009822   mary@gmail.com   A002

name   mobile     email            card_num
============================================
Peter  65666678   mary@gmail.com   A001

User.select("CASE
       WHEN (name = 'Peter') THEN 'name'
       WHEN (mobile = '74009822') THEN 'mobile'
       WHEN (email = 'mary@gmail.com') THEN 'email'
       ELSE 'card_num'
      END as col_name").where("name = 'Peter' or mobile = '74009822' or email = 'mary@gmail.com' or card_num='A002'")