MYSQL create unique id according to another column (which is a foreign key)

Question

In a DB there's a table called decks which looks like this:

ID | Name
------------
1  | Deck 1
2  | Deck 2
3  | Deck 3

And then another table called words which should look like this

Deck ID | Word ID | Text
-------------------------------
1       | 1       | Cat
2       | 1       | Elephant
1       | 2       | Dog
1       | 3       | Elephant
3       | 1       | Shark

So basically this table should have a Foreign Key which points to the deck and then a key which is set to auto-increment according to the foreign key. Also, note that the ID of this table would be the combination of Deck Id and Word Id . Is this possible in MYSQL 8?

Basically what I have is this:

CREATE TABLE my_db.words (
    deck_id INT,
    text VARCHAR(100),
    FOREIGN KEY(deck_id) REFERENCES decks(id)
);

But no idea how to create this auto-increment field.

Not sure if it's important but the text column should have no effect whatsoever in the result of this generated ID.

Answer 1

This is how you can create,

CREATE TABLE my_db.words (
    deck_id INT NOT NULL,
    word_id INT NOT NULL AUTO_INCREMENT,
    text VARCHAR(100),
    FOREIGN KEY(deck_id) REFERENCES decks(id),
    PRIMARY KEY(deck_id, word_id)
);

Answer 2

Keep 3 Tables. Fully Normalised.

Table 1 - deck Table 2 - word Table 3 - deck_word

Creating Table 1

CREATE TABLE my_db.deck (
    id INT NOT NULL AUTO INCREMENT,
    name VARCHAR(100),
    PRIMARY KEY(id)
);

Creating Table 2

CREATE TABLE my_db.word (
    id INT NOT NULL AUTO INCREMENT,
    text VARCHAR(100),
    PRIMARY KEY(id)
);

Creating Table 3

CREATE TABLE my_db.deck_word (
    deck_id INT NOT NULL,
    word_id INT NOT NULL,
    FOREIGN KEY(deck_id) REFERENCES deck(id),
    FOREIGN KEY(word_id) REFERENCES word(id),
    PRIMARY KEY(deck_id, word_id)
);

According to this both the word and the deck will be connected via the deck_word table which has the composite primary key

Answer 3

I could suggest a different form to insert into the words table:

INSERT INTO words select 1,(select count(*) from words where deckId=1), 'Cat';
INSERT INTO words select 2,(select count(*) from words where deckId=2), 'Elephant';
INSERT INTO words select 1,(select count(*) from words where deckId=1), 'Dog';
INSERT INTO words select 1,(select count(*) from words where deckId=1), 'Elephant';
INSERT INTO words select 3,(select count(*) from words where deckId=3), 'Shark';

http://sqlfiddle.com/#!9/03c119/2

This way you will get the last number, if you are making deletion this will not work.

If the orded did not matter, you can always reorder or make a virtual field to enumerate the words.