Question

I have an database table set up as shown below:

copy_blocks
id (auto-increment int)
content (text)
parent_id (int)

When a piece of copy is inserted the first time, the parent_id is set to itself. Example:

copy_blocks
id    content        parent_id
1     Hello, World   1

When that copy is updated by the user, new rows are inserted, but those row always points to the first version of the copy block.

copy_blocks
id    content         parent_id
2     Hello, World!   1
3     Heya, World!!   1

Structuring things this way allows me to either query a specific version of the content, or find the parent copy block and then look up the latest version. (In this case, version #3)

So here’s my question: Can anyone come up with a query that will always return the latest version of every content block? Ideally the query would return the results below:

id    content         parent_id
3     Heya, World!!   1

I get the feeling it would have something to do with joining against itself. I can’t think of how to do it without at least two different queries or server-side code.

Answer 1

One more option:

SELECT 
      c.*
FROM 
        ( SELECT parent_id
               , MAX(id) AS id
          FROM copy_blocks
          GROUP BY parent_id
        ) AS dc
    JOIN
        copy_blocks AS c
      ON 
        (c.parent_id, c.id) = (dc.parent_id, dc.id)