How to use Group By in SOQL ? Error : Return value must be of type: List<pba_listingobj__c>

Question

@RemoteAction
global static List<pba_listingobj__c> getAccountstages(String namejs) 
{   
 return [SELECT sales_stage__c FROM pba_listingobj__c where Account_lookup__r.Name=:namejs GROUP BY sales_stage__c];        
}

Where Account_lookup__c is Lookup to Account.

Field name: Account_lookup__c

How to have unique Stages (sales_stage__c)(values can be : Closed Won, Prospect)?

Records:

1. Prospect

2. Closed Won

3. Closed Won

4. Closed Won

Desired Output: As shown in the above screenshot.

1. Prospect

2. Closed Won

Answer 1

When you use a GROUP BY clause, the query returns List<AggregateResult>.

@RemoteAction
global static List<AggregateResult> getAccountStages(String name)
{
    // same query
}

You may wish to simply return a List<String>, depending on how you consume this data downstream.

@RemoteAction
global static List<String> getAccountStages(String name)
{
    List<String> stages = new List<String>();
    for (AggregateResult aggregate : [/*query*/])
        stages.add((String)aggregate.get('Sales_Stage__c'));
    return stages;
}