page

<apex:page controller="Enametable">
     <apex:form >
      <p/>
      Department:
      <apex:selectList value="{!Department}" multiselect="false" size="1">
          <apex:selectOptions value="{!DeptName}"/>
      </apex:selectList><p/>
      <apex:commandButton action="{!DeptName}" value="GetEmployees" reRender="out" status="mystatus"/>
     <p/>
     Employees are :
        <apex:outputPanel id="out">
        <apex:dataTable value="{!Employees}" var="emp" bgcolor="cyan" border="2">
        <apex:column headerValue="EmployeeName">
        <apex:actionSupport event="onmouseover" rerender="detail" status="Mystatus">
        <apex:param name="cid" value="{!emp.id}"/>
        </apex:actionSupport>
         {!emp.EmployeeName__c}
        </apex:column>
        </apex:datatable>
        </apex:outputpanel>
         <apex:actionstatus id="Mystatus" starttext="loadind data.....">
         <apex:facet name="stop">
        <apex:outputPanel id="detail">
        <apex:detail subject="{!$CurrentPage.parameters.cid}" relatedList="true"  title="false"/>
          </apex:outputPanel> 
          </apex:facet>
          </apex:actionStatus>

     </apex:form>
</apex:page>

Controller:

public class Enametable {

   
public PageReference DeptName()
    {
        return null;
    } 

 public String Department { get; set; }

        List<Employees__c> Employees ;
        
        public List<Employees__c> getEmployees() {
               system.debug(Department);
               if(department != null)
               {
                Department__C objdept = [select Id from Department__c where name =: department];
                system.debug(objdept.Id);
                Employees = [select EmployeeName__c from Employees__c where DeptName__c=:objdept.Id];
                return Employees;
                
             }
             return null;
        }
        
  List<Department__c> lstdept;

    public List<selectOption> getDeptName()
    {
    List<selectOption> options = new List<selectOption>();
    
    lstdept = [select Name from DepartMent__c];
    
    options.add(new SelectOption('--None--','--None--'));
    
    for(Department__c obj : lstdept)
    {
            options.add(new Selectoption(obj.Name,obj.Name));
    }
    return options;
    }

}

I worked with this code,after u have select departmentname from picklist ,click buttonthen u can get all employees who are belongs to that department then when u mousehoverd on employee name,employee detail page will displayed