[SalesForce] Bind variables only allowed in apex code

The way you are trying to do this isn't possible. Salesforce docs:

Field to field comparison in WHERE clause of SOQL

Description

I want to execute a query by comparing two fields of the same object in WHERE clause, but I am not able to use a field in the right hand side of a condition in a WHERE clause:

List<user> users = [SELECT Id,name FROM User WHERE (FirstName != Lastname)];

The above query returns: "System.QueryException: unexpected token: 'Lastname' "

Resolution

Salesforce doesn't allow direct field to field comparison in SOQL query.

To achieve this you may create a formula field that will compare fields and return a value (like true or false) which you may use in a WHERE clause.

So for the above query, you could create a formula field on User object with return type Text e.g. NameCompare, with the formula IF(User.FirstName != User.LastName, 'true', 'false')

Now our query will be:

List<User> Users = [SELECT id, name FROM User where NameCompare= 'true'];

Following idea has been posted on ideaexchange portal for allowing field-to-field comparison in SOQL:

https://success.salesforce.com/ideaView?id=08730000000BrHAAA0

In your specific scenario, you will want to create a formula field of type Checkbox like:

LastModifiedDate >  CreatedDate

Then rewrite your query with your WHERE clause like:

SELECT count() FROM Case where Last_Modified_Greater_Than_Created__c = true

You are seeing this error due to the field to field comparison in WHERE clause of SOQL.

I am not sure how you are obtaining the current Account Id but I am providing an example with this Id hardcoded (you should move away from this approach within your design) for you to move forward with.

Let's start with creating a collection of Ids to store our found "Matching" Leads (based on DunsNumer). You will need to iterate over the Account(s) (plural once you have redesigned to pass in a List of Account Ids to work with rather than the single hardocded Id) and then for each Account we can query the Leads that match on DunsNumber.

List<Id> leadsWithMatchingDUNS = new List<Id>();
for (Account acc : [select DunsNumber from Account where Id ='0012400000Max8P']) {
    Lead l = [select id from Lead where CompanyDunsNumber = :acc.DunsNumber];
    if (l != null) {
        leadsWithMatchingDUNS.add(l.Id);        
    }
}
System.debug(leadsWithMatchingDUNS);

You can follow on with this List of Lead Ids however you choose to do so. I am simply debugging for POC.

Update

After posting above solution I realized I was going against Best Practice by perform a SOQL within a for loop.

There is a governor limit that enforces a maximum number of SOQL queries. When queries are placed inside a for loop, a query is executed on each iteration and governor limit is easily reached

As of that, I have modified the code to use Map values when querying the Lead object.

Map<Id, String> accountDUNS = new Map<Id, String>();
List<Id> leadsWithMatchingDUNS = new List<Id>();

for(Account acc : [
        select DunsNumber
        from Account
        where id = '0012400000Max8P']){
    accountDUNS.put(acc.Id, acc.DunsNumber);
}

for (Lead l : [
        select id,
          CompanyDunsNumber
        from Lead
        where CompanyDunsNumber = :accountDUNS.Values()]) {  
    leadsWithMatchingDUNS.add(l.Id);
}

System.debug(leadsWithMatchingDUNS.size());