How to remove empty values in drop down using ng-option in angular js?

How to remove empty and blank values in the Drop Down using ng-option in angular js?
Some data's are in college name some of it not having the college name so what is happening in the drop down, the empty values are displaying.
I am expecting only values, not empty blank items, so how to remove empty items.
My Plunker.
And how to Mingle College Name and School Name into a single Drop Down.

My Drop Down:-

<select data-ng-model="searchtable.college_name" id="searchtable" ng-options="item.college_name as item.college_name for item in users" class="form-control" placeholder="search" required><option value="">All</option></select>

  <label for="">school name</label>
       <select data-ng-model="searchtable.school_name" id="searchtable" ng-options="item.school_name as item.school_name for item in users" class="form-control" placeholder="search" required><option value="">All</option></select>

My Html:-

<div class="col-md-3">
    <label for="">College name</label>
    <select data-ng-model="searchtable.college_name" id="searchtable" ng-options="item.college_name as item.college_name for item in users" class="form-control" placeholder="search" required>
        <option value="">All</option>
    </select>
    <label for="">school name</label>
    <select data-ng-model="searchtable.school_name" id="searchtable" ng-options="item.school_name as item.school_name for item in users" class="form-control" placeholder="search" required>
        <option value="">All</option>
    </select>
    <div ng-repeat="question in users | filter: searchtable | filter: myrole"> <small>
                  <table border="0">
                    <tbody>
                      <th>{{question.displayName}}</th>
                    <th style="background: yellow;">,{{question.roles[0]}}</th>
                    <th style="background: light;">,{{question.request_role[0]}}</th>

                    </tbody>
                          </table>


                  </small> </div>
    <p>How to mingle college name and school name into single drop down:-</p>
    <p>And how to do filter college name and school name while selecting the filed</p>
</div>

My Data:-

    $scope.users = [{
"_id": "59a6c7c96c2ce324124cc1d8",
"displayName": "blink foundation",
"provider": "local",
"location": "icf",
"username": "blink",
"dob": "1991-10-04T18:30:00.000Z",
"phone": 7299345250,
"religion": "Hindu",
"college_name": "Arignar Anna",
"__v": 2,
"created": "2017-08-30T14:12:25.397Z",
"roles": [
"block"
],
"request_role": [
"Change Agent"
],
"lastName": "foundation",
"firstName": "blink"
},
{
"_id": "598abfe4cce8ed582a2d8b32",
"displayName": "avinaash muthukumar",
"provider": "local",
"username": "avinaash muthu",
"__v": 0,
"created": "2017-08-09T07:55:16.511Z",
"roles": [
"admin"
],
"request_role": ["Change Agent"],
"firstName": "avinaash"
},
{
"_id": "5979a591c999e9302caece13",
"displayName": "Ajmal Afthab",
"provider": "local",
"location": "Urapakkam",
"username": "ajmal_afthab",
"dob": "1995-01-23T18:30:00.000Z",
"phone": 9500269131,
"religion": "Islam",
"school_name": "public school",
"__v": 1,
"roles": [
"kp"
],
"request_role": ["school student"],
"categories": [
"Religion & Culture",
"Moral Ethics",
"Social Psychology"
],
"lastName": "Afthab",
"firstName": "Ajmal"
},
{
"_id": "5978a2886d5b10ec321a2114",
"displayName": "happy selvam",
"provider": "local",
"username": "happy",
"__v": 0,
"created": "2017-07-26T14:09:12.730Z",
"roles": [
"admin"
],
"request_role": ["parent"],
"categories": [],
"lastName": "selvam",
"firstName": "happy"
},
{
"_id": "58e73c5c9e2f3f1421e241be",
"displayName": "sarawana kumar",
"religion": "hindu",
"college_name": "IIT",
"__v": 2,
"roles": [
"user"
],
"request_role": ["school student"],
"categories": [
"Religion & Culture",
"Social Psychology"
],
"lastName": "kumar",
"firstName": "sarawana"
},
{
"_id": "58d0fab62758cc482c295eba",
"displayName": "avinaash kumaran",
"provider": "local",
"username": "avinaash",
"roles": [
"block"
],
"request_role": ["parent"],
"categories": [],
"lastName": "kumaran",
"firstName": "avinaash"
},
    ]

Solution

You can use a filter (or write a custom filter) to remove the blank options for your first case.

HTML:

<select data-ng-model="searchtable.college_name" id="searchtable" ng-options="item.college_name as item.college_name for item in users | filter:{ college_name :'!!'}" class="form-control" placeholder="search" required>
    <option value="">All</option>
</select>
<label for="">school name</label>
<select data-ng-model="searchtable.school_name" id="searchtable" ng-options="item.school_name as item.school_name for item in users | filter:{ school_name :'!!'}" class="form-control" placeholder="search" required>
    <option value="">All</option>
</select>

Now for your second case, if you want to combine both school and colleges into one dropdown, just add the following. Use a custom filter to return non empty values.

HTML

<select data-ng-model="searchtable.school_name" id="searchtable" ng-options="item.college_name +''+ item.school_name for item in users|filter:notBlank" class="form-control" placeholder="search" required><option value="">All</option></select>

JS:

 $scope.notBlank= function(item){
   return (item.college_name || item.school_name)
}

Working Plunker: http://plnkr.co/edit/QF5rnghuIWfcdeQdbAvd?p=preview