Avoid duplicate data in mySQL table - mysql

Question

I have the following table in my DDBB:
CREATE TABLE subjects (
subject_id int(11) NOT NULL AUTO_INCREMENT,
subject text,
PRIMARY KEY (subject_id, subject)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 AUTO_INCREMENT=1;
This is an example of my table:
id | subject |
1 test
2 ICT
3 ICT
The key (id) is not duplicate since it is automatically generated by MySQL, but the las two rows are repeating.
How can I avoid repeating the subject name?
I have read that it can be done with a 'constraint' like this:
ALTER TABLE subjects
ADD CONSTRAINT constraint_subject UNIQUE KEY(subject);
But I've tried it and I get an error every time.
I know that this has been asked before but I'm still entering from my PHP, some subjects with the same name and the program always lets me enter them.

Answer 1

Adding the constraint gives you an error because it wouldn't be satisfied since you already have duplicated data.
You have to delete the duplicates and then add the constraint, which would then work.
If you want to select only distinct rows right now, even when having duplicates in your database, you can run the following:
SELECT
*
FROM
subjects AS s1
WHERE
NOT EXISTS (
SELECT
id
FROM
subjects AS s2
WHERE
s1.subject = s2.subject
AND s1.id != s2.id
);
Or
SELECT
s1.*
FROM
subjects AS s1
LEFT JOIN
subjects AS s2
ON (s1.subject = s2.subject AND s1.id != s2.id)
WHERE
s2.id IS NULL
Both will give the same result but I find the first one to be more explicit about what you're trying to achieve.

Answer 2

You can't create an index on a column of data type TEXT because that's too long for an index.
You can create an index, even a unique index, on a prefix of that column.
ALTER TABLE subjects
ADD CONSTRAINT constraint_subject UNIQUE KEY(subject(191));
This means two subjects cannot have exactly the same leading 191 characters.
I don't think you should declare the PRIMARY KEY including the subject. It's more typical to use the auto-increment integer column alone as the primary key.
So your table ends up with this definition:
CREATE TABLE `subjects` (
`subject_id` int(11) NOT NULL AUTO_INCREMENT,
`subject` text,
PRIMARY KEY (`subject_id`),
UNIQUE KEY `constraint_subject` (`subject`(191))
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
I chose the length 191 because it's the longest I could fit in the 767 byte limit that InnoDB has on indexes (utf8mb4 characters count as 4 bytes).