MySQL Make a combination of columns unique -

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i have table stores comments users make images on site. table structured 4 columns, row_id, primary key, image_id, user_id , comment. want ensure user can leave 1 comment per image. create unique index on 2 columns?

create unique index imgusr on comments (image_id, user_id);

the idea following query work:

insert comments set image_id = '1', user_id = '2', comment = 'nice' on duplicate key update comment = 'nice';

the gotchya (gotme?) table innodb because expected large. approach work, despite presence of primary key?

yes work perfectly.

in topic, why did have row_id ? can put primary key (image_id, user_id), works too.


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