Question

So I have two tables like this:

create table A
{
    id int;
    ...
}

create table B
{
    id int;
    a_id int;
    t timestamp;
    ...
}

A is one-to-many with B

I want to:

SELECT * FROM A LEFT JOIN B ON A.id = B.a_id ???

But I want to return exactly one row for each entry in A which has the B with the newest t field (or null for Bs fields if it has no B entry).

That is rather than returning all A-B pairs, I want to only select the newest one with respect to A (or A-null if no B entry).

Is there some way to express this in SQL? (I’m using MySQL 5.5)

Answer 1

LEFT JOIN is only concerned with ensuring every row in A is returned, even if there is no corresponding joined row in B.

The need for just one row needs another condition. MySQL is limitted in its options, but one could be:

SELECT
  *
FROM
  A
LEFT JOIN
  B
    ON  B.id = A.id
    AND B.t = (SELECT MAX(lookup.t) FROM B AS lookup WHERE lookup.id = A.id)

Another could be…

SELECT
  *
FROM
  A
LEFT JOIN
(
  SELECT id, MAX(t) AS t FROM B GROUP BY id
)
  AS lookup
    ON lookup.id = A.id
LEFT JOIN
  B
    ON  B.id = lookup.id
    AND B.t  = lookup.t