Prevent users from booking multiple events

Hi 
I have a situation whereby I  have 3 events running in 1 day
Category = Meeting 1 
Events = Event1, Event2, Event3 
I want users to register then login and only be allowed to book 1 of the 3 events (so they don't get greedy and pick and choose which event they want to attend therefore preventing others from booking)
I have tried a Custom sql field with: With the aim of returning 0 or 1 
I could then validate on that value.
The problem i have is the Custom sql field renders as a dropdown.
Is there any other way of doing this?
Thanks

Hi,
I am trying to use the validator.php as per eventbookingdoc.joomservices.com/basic-s...eld-data-server-side to prevent users from booking more than 1 event. see attached file (I cant post the code into the forum due to Sucuri blocking )
I have created a Custom field in EB called "check email" for testing purposes.
If the user enters the same email that they used to book a previous event (id = 2027)it should return False.
I am using the code in the google doc - it doesnt throw any errors but it still allows the booking to go through.
The user has already booked onto event with id 2027 so it should return false.
I can confirm I am loading my_custom_rule into the  Server Validation Rules field
Any pointers please?
docs.google.com/document/d/15sDRNK9yqXYs...yg8/edit?usp=sharing  

Could you please use this updated validator.php file which I upload here? I don't see anything wrong from the original code except the where clause needs to be changed abit

 

Thanks Tuan,
I needed to validate against the select count so if it was 0 return false.
I find the j! sql queries tricky so my solution now looks like this.
To save having to enter loads of event ids i did a join on the category field which means if an event has already been booked in that category then disallow a second booking.
I am using a Custom field > text with a  client side validation of validate[equals

Thanks. Please just leave the code there. If someone needs it, they can use it (or at least uses it as base code and customize it further to meet their own need)

Regards,

Tuan