Question

I have the following problem to solve:

Let’s say there is a table A that contains an number of elements, this table is copied to become table B. In table B some of the original elements are lost and some are added. A reference table AB keeps track of these changes. Then table B is copied to be table C and again some of the existing elements get lost and some are added. Another reference Table BC keeps track of these relations. … etc.

There is an n number of such tables with an n-1 number of reference tables.

If I want to know which of the elements of my choice in table C where already present in A, I can do that by doing something like:

SELECT AB.oldID
FROM AB
JOIN BC
WHERE BC.newID IN (x, y, z)

Now since the number of reference tables can wary, the number of JOIN lines can wary.

Should I concatenate the query by looping over the steps and adding JOIN lines or shoudl I rather write a recursive function that selects only the members of the next step and then let the function call itself until I have the end result?

Or is there an other even better way to do something like that?

Answer 1

Since your table names vary, you’ll need to build some kind of a dynamical query.

If you do the recursive function approach, you’ll need to pass the resultsets between the function calls somehow.

MySQL has no array datatype, and storing the results in a temp table is way too long.

Conclusion: use joins.

Update:

Here’s a sample query which returns the entries that persisted through revision A to revision M (with one table design):

SELECT  *
FROM    entries e
WHERE   NOT EXISTS
        (
        SELECT  *
        FROM    revisions r
        JOIN    revision_changes rc
        ON      rc.revision_id = r.id
        WHERE   rc.entry_id = e.id
                AND rc.deleted
                AND r.revision_id BETWEEN 'A' AND 'M'
        )

This way, you just fill the added and deleted fields of revision_changes for the revisions where the entry was added or deleted.