Question

I have a table that holds a users favorite categories and I was wondering if My MYSQL tables structure is built correctly to hold the MySQL data provided correctly?

MySQL data.

userID      catID
4       21
4       4
4       67
3       34
3       4

MySQL table .

CREATE TABLE ab (
userID INT UNSIGNED NOT NULL, 
catID INT UNSIGNED NOT NULL,
PRIMARY KEY (userID),
UNIQUE KEY (catID)
);

Answer 1

Neither userID nor catID are by themselves unique. What you want is

CREATE TABLE ab (
    userID INT UNSIGNED NOT NULL, 
    catID INT UNSIGNED NOT NULL,
    PRIMARY KEY (userID, catID)
);

so that only the specific combinations of userID and catID taken together are required to be unique.