Question

Is there a way to delete duplicate records based on two fields?

I have a system where people can register for sport events. In the table:

event_registrations
• unique_id
• eventname
• id (person’s id number)
• Name and Surname

One person can apply for many events – id may duplicate
an event may have multiple participants – eventname may duplicate:

--Johnsmith--  --Mountain Cycle--
--Johnsmith--  --Marathnon Walk--

--Linda--      --Mountain Cycle--
--Johnsmith--  --Mountain Cycle--

But a person may not register for a event they have already registered for:

--Johnsmith--   --Mountain Cycle--
--Johnsmith--   --Mountain Cycle--

They Select a event name through a form. Then the form data and their user details is stored in table event_registrations.

Any help would be appreciated

Answer 1

First delete any rows with duplicate (eventname, id) combinations.

Then add the UNIQUE constraint:

ALTER TABLE yourTable
  ADD CONSTRAINT eventname_person_Unique
    UNIQUE INDEX eventname_id_U 
       (eventname, id) ;

Your form that adds registrations should be adjusted accordingly to treat the error it will get from MySQL when a duplicate row is rejected.