Question

I have data that looks like this:

entities id         name 1          Apple 2          Orange 3          Banana 

Periodically, a process will run and give a score to each entity. The process generates the data and adds it to a scores table like so:

scores  id  entity_id    score   date_added 1    1            10       1/2/09 2    2            10       1/2/09 3    1            15       1/3/09 4    2            10       1/03/09 5    1            15       1/4/09 6    2            15       1/4/09 7    3            22       1/4/09 

I want to be able to select all of the entities along with the most recent recorded score for each resulting in some data like this:

entities id name     score  date_added 1  Apple     15     1/4/09 2  Orange    15     1/4/09 3  Banana    15     1/4/09 

I can get the data for a single entity using this query:

SELECT entities.*,         scores.score,         scores.date_added  FROM entities  INNER  JOIN scores ON entities.id = scores.entity_id  WHERE entities.id = ?  ORDER BY scores.date_added DESC LIMIT 1 

But I’m at a loss for how to select the same for all entities. Perhaps it’s staring me in the face?

Thank you very kindly for taking the time.

Thanks for the great responses. I’ll give it a few days to see if a preferred solution bubbles up then I’ll select the answer.

UPDATE: I’ve tried out several of the proposed solutions, the main issue I’m facing now is that if an entity does not yet have a generated score they don’t appear in the list.

What would the SQL look like to ensure that all entities are returned, even if they don’t have any score posted yet?

UPDATE: Answer selected. Thanks everyone!

Answer 1

I do it this way:

SELECT e.*, s1.score, s1.date_added  FROM entities e   INNER JOIN scores s1     ON (e.id = s1.entity_id)   LEFT OUTER JOIN scores s2     ON (e.id = s2.entity_id AND s1.id < s2.id) WHERE s2.id IS NULL;