Prevent duplicate values in database - mysql

Question

I have one table that store values about recurrence of some events, for example, the event with id=1 occurs in Monday, Wednesday and Friday, and the event with

Answer 1

Mr. Linoff provided the right way but if you cant's modify your schema, you can also check on insert with row subquery:

insert into table_name
select 1, 2, 'Monday' from dual
where (1,2, 'Monday') not in
(select id,event_id, day from table_name);

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Or use where not exists

insert into table_name
select 1, 2, 'Monday' from dual
where not exists
(select 1,2, 'Monday' from table_name);

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