diff --git a/highscores_database.sql b/highscores_database.sql
index 6e4be47..1854cbf 100644
--- a/highscores_database.sql
+++ b/highscores_database.sql
@@ -50,5 +50,6 @@ CREATE TABLE Submissions(
    GuildId int unsigned not null,
    MessageId int unsigned not null,
    Score varchar(30) not null, -- Score is stored as a string until converted during submission
-   UserName varchar(30) not null
+   UserName varchar(30) not null,
+   PRIMARY KEY (CategoryId, UserName) -- one submission a category per user.
 );
\ No newline at end of file