I have 2 custom object.
1 - namespace__object1__c
2 - namespace__object2__c
I have lookup relationship in custom object namespace__object2__c.
Now i want to query id record from object2 where object1 id = (provided parameter)
String s = 'a6T3k0000LMJcEAO';
string query = 'select ID, (select ID from namespace__object1__c where ID = :s ) from namespace__object2__c';
SObject rObject = database.query(query);
system.debug(rObject);
Added:- defined relation
object1 namespace__object1__c Lookup(object)