How to design one user "likes" another? - mysql

Question

I would imagine this is kind of like friends, but doesn't need to be two-way. So like this:
from_user to_user
-------------------
John Mary
Mary John
John Susan
Mary Dave
I was curious if there were any issues with doing it as shown below.
user1 user2 mutuality
--------------------------------
John Mary 2
John Susan 1
Mary Dave 1
The second seems like it would have more complicated queries and checks, but wouldn't it end up saving space? For example, if Susan ends up falling for John one day, you'd check user2 for Susan and user1 for John. If it exists, update the mutuality to 2. If not, insert a new row [Susan, John, 1]. Something like that?
What's the best way of doing this?

Answer 1

I would choose the first method. You will not end up saving enough space with the second method to be an issue. Since each choice (like) is an action, having a separate record to store that action matches the work flow. John likes Mary, add a record. Mary likes John, add another record. Mary no longer likes John, delete that single record. This is easier to maintain (for me). I find keeping things in smaller granular parts keeps it simple.
In terms of design, I would recommend that instead of using string names in the likes table, you should have a users table with each user have a primary key id (unique). This way, users can modify names without breaking the relationships.
The table data would look like this:
Users table
id Name
1 John
2 Mary
3 Susan
4 Dave
Likes table
From To
1 2
2 1
1 3
2 4
The tables would have the following structure
users
-----
id (integer autoincrement)
name (varchar)
likes
-----
id (integer autoincrement)
fromid (integer)
toid (integer)
The sql would look like this to produce the list
SELECT f.name as fromName, t.name as toName
FROM (likes JOIN users AS f ON likes.fromid = f.id)
JOIN users AS t ON likes.toid = t.id;

Answer 2

The second method allows the following anomaly:
user1 user2 mutuality
--------------------------------
John Mary 2
Mary John 1
Do John and Mary like each other mutually (as implied by the first row), or just Mary likes John uni-directionally (as implied by the second row)?
The first method doesn't suffer from such inconsistencies, and data integrity is generally more important than saving some space.
BTW, if you introduce an integer surrogate key and reference it from the junction table (instead of the actual names), the junction table will become much leaner, negating much of the space advantage of the second method.