问题
Well, i have a table with 4 columns(id, event_id, group_id, isbn), where id is PK, event_id and group_id are FK's, and my problem is:
I need the isbn number to be unique for each event_id, let me give you some examples of some rows that should be possible and some that should not:
id | event_id | group_id | isbn
(1,1,1,123) ok
(2,1,2,123) ok
(3,1,4,123) ok
(4,1,7,1234) ok
(5,2,8,123) NOT OK, the 'isbn' must be unique for event_id('123' was already used in the first row with event_id = 1)
the group_id only appears once for each event_id, but if i make a unique contraint with the 3 columns i would be able to repeat the isbn just chaning the event_id, and i don't want that,once an isbn is used in an event_id it cant appear in another event_id, an event_id(let's say '1') can repeat the same ibsn as longe as it needs for each group_id
I know i kind of repeated the problem several times, but is a tricky question and i want to lower the chances of getting wrong answers
EDIT1: about @Andomar answer, the isbn must be related to the group_id by the cardinality (1,n) 1-isbn -> n-group_id and the structure in the answer don't do that
回答1:
What you describe is that event_id is dependant on the isbn . You need to normalize the table by splitting it into two:
(Corrected):
Remove Keep isbn in this - and add a FOREIGN KEY (event_id, isbn) constraint to the second table, below:
id | event_id | group_id | isbn
1 1 1 123
2 1 2 123
3 1 4 123
4 1 7 1234
5 2 8 123 --- not allowed by the FK constraint
And create a new table with isbn as the primary key and a UNIQUE (event_id, isbn) key (and two foreign keys: event_id to Event table and isbn to (Book?), if you have a table where isbn is the primary or unique key):
event_id | isbn
1 123
1 1234
回答2:
To keep the (event_id, isbn) combination unique:
create unique index UX_YourTable_EventIsbn on YourTable(event_id, isbn)
Per the comment, if you'd like to have only one event per isbn, you could redesign the table relation. Create a table called isbn that has a a primary key isbn and a field called event_id. That way, an ISBN can have zero or one events associated with it.
isbn isbn primary key, event_id
other_table id, event_id, group_id
A query would look up the isbns for an event in the isbn table.
来源：https://stackoverflow.com/questions/13074400/mysql-unique-constraint-multiple-columns-condition