The MySQL UNIQUE constraint is often used in a
  column definition in which we need each value for that column to
  be distinct from the others. Perhaps it is an email column for an
  on-line registration form and we want to ensure that users cannot
  register twice for an account using the same email. Whatever the
  case may be, UNIQUE is there to help us ensure this
  type of data integrity or business requirement. What if the
  target table already exists and you determine you need to add a
  UNIQUE constraint to an existing column? In this
  post, I will cover 2 ways you can implement a UNIQUE
  constraint on existing columns using the phpMyAdmin web
  interface…
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