MySQL primary key that I never select - mysql

Question

Quite often I encounter situation like this:
table `user_adress`
+----------+-------------+--------------+---------+
|adress_id | user_id | adress_type |adress |
+----------+-------------+--------------+---------+
| 1 | 1 | home |adressXXX|
| 2 | 2 | home |adressXXX|
| 3 | 3 | home |adressXXX|
| 4 | 1 | work |adressXXX|
| 5 | 2 | work |adressXXX|
| 6 | 1 | second_home |adressXXX|
+----------+-------------+--------------+---------+
If I want to use it, I'm using queries like this:
SELECT `adress` FROM `user_adress` WHERE `user_id`=1;
Seems quite normal, but the thing is, that I use "useless" adress_id column, that has no other purpouse but to be an primary key with autoincrement just for the sake of having an primary key in MySQL table. I never use or need this number. So I figured out that I should not use primary key in my table at all, remove totally adress_id, and set INDEX (without unique) at user_id column. That seems to be good - or am I wrong?
I have some doubts, because as much as I'm reading, everywhere I see advices, that every table should, or even need to have primary key. But why? Perhaps my database is badly designed if I allowed this to happen, but looking on my extreamly simple example table - I can't imagine how this could be the case in every situation, especially in such simple cases. I deffinetly missunderstanded some simple, basic rules about creating tables and properly indexing them - where is the hole in my toughts?

Answer 1

Purely based on your table structure, I would say that your primary key is incorrect.
Instead, it looks like your primary should be:
PRIMARY KEY (user_id, address_type)
You are correct that every table should have a primary key ideally, but primary keys can be over multiple fields.
It is still sometimes easier to have a simple auto-incrementing id as your primary key. The Innodb storage engine will actually do this secretly in an invisible field.
Maybe in your limited example it's not needed, but in a lot of real-world cases it can just make it easier to work with the data. In that sense I would say that having an artificial auto-incrementing primary key is not a best practice from an academic standpoint, but it can be good idea from a 'real world, operational, and MySQL admin' perspective.
There's also ORM systems out there that simply require this (bad as that is).

Answer 2

As is evident in your data the primary key allow the access directly to a single row without any problem or ambiguity .. (expecially for delete or updated)
this is specifically the purpose of a primary key ..
di the fact you could need join this table to others table by user_id
and index (not unique ) on user_id
create index myidx on mytable(user_id)
is really useful for faster join allow a direct access only at the rows related to a single user_id

Answer 3

It's true that a relational database table needs a primary key.
But it all comes down to the definition of a primary key. A primary key is NOT necessarily a single integer column that auto-increments.
A primary key is any column or set of multiple columns that can uniquely identify every row. In your case, the combination of user_id and address_type can do this (as Evert posted already).
So if you make your table like this:
CREATE TABLE user_address (
user_id INT NOT NULL,
address_type varchar(10) NOT NULL,
address TEXT NOT NULL,
PRIMARY KEY (user_id, address_type)
);
Then you can update or delete one specific row at a time like this:
UPDATE user_address SET ...
WHERE user_id = ? AND address_type = ?;
Some people feel that it's more convenient to enforce a convention that every table should have a single integer column as its primary key. They even may insist that the column must be called id for the sake of consistency.
There's some advantage in consistency, but on the other hand, it's kind of brainless to insist on that convention even when it's not helpful.