Question

The problem:
I have a table that records data rows in foo. Each time the row is updated, a new row is inserted along with a revision number. The table looks like:

id  rev field
1   1   test1
2   1   fsdfs
3   1   jfds
1   2   test2

Note: the last record is a newer version of the first row.

Is there an efficient way to query for the latest version of a record and for a specific version of a record?

For instance, a query for rev=2 would return the 2, 3 and 4th row (not the replaced 1st row though) while a query for rev=1 yields those rows with rev <= 1 and in case of duplicated ids, the one with the higher revision number is chosen (record: 1, 2, 3).

I would not prefer to return the result in an iterative way.

Answer 1

To get only latest revisions:

SELECT * from t t1
WHERE t1.rev = 
  (SELECT max(rev) FROM t t2 WHERE t2.id = t1.id)

To get a specific revision, in this case 1 (and if an item doesn’t have the revision yet the next smallest revision):

SELECT * from foo t1
WHERE t1.rev = 
  (SELECT max(rev) 
   FROM foo t2 
   WHERE t2.id = t1.id
   AND t2.rev <= 1)

It might not be the most efficient way to do this, but right now I cannot figure a better way to do this.