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

Ahh, Salesforce doesn't allow direct field to field comparisons in a query. Source:

Field to field comparison in WHERE clause of SOQL

Knowledge Article Number

000187460

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 does not 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 Checkbox e.g. NameCompare, with the formula

User.FirstName != User.LastName

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