Ideal data structure for accessing related elements in a set - php

Question

Let's say I have two sets
Set 1: A, B, C
Set 2: X, Y, Z
When element B is accessed we should also retrieve A and C together with B, the other elements in Set 1
I can achieve this with permuting each set and creating an entry in a hashtable for each element of the set and storing/copying all values available in that set, such as;
A: A, B, C
B: A, B, C
C: A, B, C
But that has a memory cost and can become inefficient for large data sets. I'm trying to achieve this without going for a graph database. Data is stored in MySQL. Any suggestions?

Answer 1

For example, you can to store pointers to identifiers of elements (or identifiers of abstract "couples" of elements (which are not really exist in database), if you will use the second way) in dataset in database for each data row. Then, in PHP, you should to build an array like this:
array(
'id1' => array(
'value' => 'A',
'links' => array('id2', 'id3')
),
'id2' => array(
'value' => 'B',
'links' => array('id1', 'id3')
),
'id3' => array(
'value' => 'C',
'links' => array('id1', 'id2')
)
);
Or, you can to mark couple of (A, B, C) with an identifier and than got an array like this:
array(
'couples' => array(
'c1' => array('id1', 'id2', 'id3')
),
'values' => array(
'id1' => array(
'value' => 'A',
'links' => array('c1')
),
'id2' => array(
'value' => 'B',
'links' => array('c1')
),
'id3' => array(
'value' => 'C',
'links' => array('c1')
)
)
);
Then, just write simple function to get all linked elements. In first case, you must to loop over links and simply get item from current array by current key, in second case, you must to loop over links, and get all elements from current couple by key in couples, and get all items from it.

Answer 2

I think you've just reached the chief driving force behind the creation of NOSQL DB's and SQL alternative :-)
There are two way to get this done with MySQL (assuming sets of arbitrary length) :
1- Have a reference on each set in your DB and use its unique identifier as a column in the item tables, it should be something like this :
id set_id name whatever_col1 whatever_col2
1 1 A ... ...
2 1 B ... ...
3 1 C ... ...
4 2 X ... ...
5 2 Y ... ...
...
Or create a key-pairs relation table having the couples (A,B), (A,C), (B,C) (X,Y), ...