code: function convertPrimaryToUnique (model, column) {
      // build query
      var sql = `ALTER TABLE ${quoteName(model.name)} DROP PRIMARY KEY, ADD UNIQUE KEY (${quoteName(column.name)})`
      // reture sql string
      return sql
  }
docs: @function convertPrimaryToUnique
  
  generate sql to convert primary key to unique index
  
  @param {ImmutableCoreModel} model
  @param {object} column
  
  @returns {string}
  
  @throws {Error}