title: How to put validation in mat-select
body: <p>I have multiple rows with mat-select.I want to be able to put validation such a way that I should be able to choose minimum of 1 value each.If I don't select 1 value of each type from mat-select then i should not be able to proceed to next step.</p>
  
  <pre><code>&lt;ng-container matColumnDef="type"&gt;Type
              &lt;mat-header-cell *matHeaderCellDef mat-sort-header&gt;Type&lt;/mat-header-cell&gt;
              &lt;mat-cell *matCellDef="let element"&gt;
                &lt;mat-select placeholder="Select Type" [(ngModel)]="type.text" (selectionChange)="changed($event)"&gt;
                  &lt;mat-option *ngFor="let type of typeColumn  let i = index" [value]="{type: type.text, index: i}" [disabled]="!type.allowed &amp;&amp; type.allowed != undefined"&gt;
                    {{ type.text }} 
                  &lt;/mat-option&gt;
                &lt;/mat-select&gt;
              &lt;/mat-cell&gt;
            &lt;/ng-container&gt;
  </code></pre>
  
  <p>This html code allows me to display mat-select and displays list of dropwdown values.</p>
  
  <pre><code>  typeColumn = [
  {text:'None'},
  {text:'Time',allowed: true},
  {text:'Segment'},
  {text:'Key',allowed: true},
  {text:'Input'},
  {text:'Quantile' } ];
  </code></pre>
  
  <p>This is the list of drop down options that i get.</p>
  
  <p>I need a validation that the user should select at least one value of each Time,Key,Segment,Input and Key before proceeding to next step.</p>