Error: Entity 'Task' is not supported for semi join inner selects

Question

I am not able to make this query work, I don't know how to use the WHERE NOT IN clause. Can anyone please help me with this? I will appreciate any help. Thank you!

First approach:

 existingTasks = [Select Subject From Task Where WhatId IN :  existingServTaskLibrary  ];

 for(Service_Task_Library__c ta: [SELECT Id, Name  from 
   Service_Task_Library__c  WHERE Service__c IN : selectedServices  And 
   Subject__c NOT IN: existingTasks  Order by Service__c ASC]) 
   {
     .. do something

    } 

Error first approach

Invalid bind expression type of Task for column of type String

Second Approach

 for(Service_Task_Library__c ta: [SELECT Id, Name  from 
   Service_Task_Library__c  WHERE Service__c IN : selectedServices  And 
   Subject__c NOT IN (Select Subject From Task Where WhatId IN : existingServTaskLibrary) 
   Order by Service__c ASC]) 
   {
     .. do something

    } 

Error for second approach:

Error: Entity 'Task' is not supported for semi join inner selects

Answer 1

Mistake in First approach: Here you are using existingTasks which i assume list of task. When you use this as Subject__c not in : existingTasks, Soql compiler thinks existingTasks as list of String but it is list of Task. So only you got the error.

Suggestion: After existingTasks query add this.

Set<string> subjectSet = new Set<string>();
for(Task t : existingTasks){
    subjectSet.add(t.Subject);
}

Use the new Set in for each loop as

Subject__c NOT IN: subjectSet

If you succeed in this, there won't be a need for second approach ;)

Answer 2

I think pulling out the Task part of the query won't affect the efficiency of your existing for(...[Select]) statement.

I'd just query your subject list prior to the for loop:

Task[] tasks = [Select Subject From Task Where WhatId IN :existingServTaskLibrary];

Then you can just include this tasks variable in your NOT IN clause.

---

EDIT

Oh and yes, I didn't look at the first approach - and the fix suggested by Vigneshwaran G is correct - you need a list of ids, not a list of sObjects.

One improvement would be to enclose that query in a Map to extract the ids without a loop - ie:

Map<Id,Task> taskMap = new Map<Id,Task>([Your query here]);
Set<Id> taskSet = taskMap.keySet();

Or even faster:

Set<Id> taskSet = new Map<Id,Task>([Your query here]).keySet();