Cannot Add Registration to cart: SQL Error with Registration and Drupal Commerce I have drupal site running core 7.42 and Commerce 7x-1.11 with Commerce Registration and Entity Registration. The modules all seem to be working fine as I can create registration products and product displays and add registrations to pages and such. 

The problem is when a user tries to add the registration to cart it produces the following error

       PDOException: SQLSTATE[42000]: Syntax error or access violation: 
       1064 You have an error in your SQL syntax; check the manual that 
       corresponds to your MySQL server version for the right syntax to use near
        '))  LIMIT 1 OFFSET 0' at line 1: SELECT registration.registration_id  
        AS entity_id, registration.type AS bundle, :entity_type AS  entity_type, 
NULL AS revision_id FROM {registration} registration WHERE (registration.entity_id 
         = :db_condition_placeholder_0) AND (registration.user_uid = :db_condition_placeholder_1) 
     AND (registration.state IN ()) LIMIT 1 OFFSET 0; Array ( [:db_condition_placeholder_0] =&gt; 5 
        [:db_condition_placeholder_1] =&gt; 1522 [:entity_type] =&gt; registration ) in EntityFieldQuery-&gt;execute() 
    (line 1192 of /home/phislub9/public_html/includes/entity.inc).


I'm not sure what is causing the error, or how to debug it. It looks like a bad query is getting passed to the a drupal_alter function to modify query's in entity.inc.
So, the error was the result of missing registration states. I found the details in this post on the Entity Registration Issue Queue Pdoexception Error. I added a default "active" registration state, and this solved the problem.