问题
I have two tables --> Variables (id, name) and Variable_Entries (id, var_id, value).
I want each variable to have a unique set of entries. If I make the value entry unique then a different variable won't be able to have that same value which is not right.
Is there some way to make the value column unique for identical var_id's?
回答1:
Yes:
alter table Variable_Entries add unique (var_id, value);
Now you have a unique constraint across var_id and value together. In other words, no occurrence of var_id and value can appear more than once.
回答2:
Yes, you can create a composite unique key:
ALTER TABLE variable_entries ADD UNIQUE (var_id, value);
回答3:
Add a unique key of Variable_Entries for the combined fields var_id/value.
Also, you should always use singular words for table names (user instead of users). Never use uppercase characters in the table name, because that will cause you a NIGHTMARE over different operating systems.
来源：https://stackoverflow.com/questions/2504007/mysql-unique-constraint-on-multiple-fields