Angularjs md-select multiple md-optgroup

  <md-select ng-model="selection" placeholder="Select an Option">
      <md-optgroup label="{{currentGroup}}" ng-repeat="currentGroup in groupList">
        <md-option ng-value="item" ng-repeat="item in items | filter: {group: currentGroup}">{{item.name}}</md-option>
      </md-optgroup>
    </md-select>

app.controller('MainCtrl', function($scope) {

  $scope.desc = "Angularjs md-select multiple md-optgroup";

  $scope.items = [{
    name: 'SFTP',
    group: 'Text File'
  }, {
    name: 'FTP',
    group: 'Text File'
  }, {
    name: 'MYSQL',
    group: 'RDBMS'
  }, {
    name: 'Oracle',
    group: 'RDBMS'
  }, {
    name: 'Sybase',
    group: 'RDBMS'
  },
  {
    name: 'DB2',
    group: 'RDBMS'
  }];


// Here I'm just calculating the groups when the controller runs
// You would probably want to use a $watch if you expected the list to change
  $scope.groupList = $scope.items.reduce(function(previous, current) {
    if (previous.indexOf(current.group) === -1) {
      previous.push(current.group);
    }
    //console.log(previous);
    return previous;
  }, []);

});

Reference :- http://plnkr.co/edit/ApuUq7V88ad6uplKjKSC?p=preview&preview