Question

I have a table defined like this:

CREATE TABLE mytable (id            INT NOT NULL AUTO_INCREMENT, PRIMARY KEY(id), 
                      user_id       INT REFERENCES user(id) ON UPDATE CASCASE ON DELETE RESTRICT,
                      amount        REAL NOT NULL CHECK (amount > 0),
                      record_date   DATE NOT NULL
                     );
CREATE UNIQUE INDEX idxu_mybl_key ON mytable (user_id, amount, record_date);

I want to write a query that will have two columns:

• user_id
• amount

There should be only ONE entry in the returned result set for a given user. Furthermore, the amount figure returned should be the last recoreded amount for the user (i.e. MAX(record_date).

The complication arises because weights are recorded on different dates for different users, so there is no single LAST record_date for all users.

How may I write (preferably an ANSI SQL) query to return the columns mentioned previously, but ensuring that its only the amount for the last recorded amount for the user that is returned?

As an aside, it is probably a good idea to return the ‘record_date’ column as well in the query, so that it is eas(ier) to verify that the query is working as required.

I am using MySQL as my backend db, but ideally the query should be db agnostic (i.e. ANSI SQL) if possible.

Answer 1

First you need the last record_date for each user:

select user_id, max(record_date) as last_record_date
from mytable
group by user_id

Now, you can join previous query with mytable itself to get amount for this record_date:

select 
      t1.user_id, last_record_date, amount
from 
      mytable  t1
  inner join
      ( select user_id, max(record_date) as last_record_date
        from mytable
        group by user_id 
      ) t2
    on  t1.user_id = t2.user_id 
    and t1.record_date = t2.last_record_date

A problem appears becuase a user can have several rows for same last_record_date (with different amounts). Then you should get one of them, sample (getting the max of the different amounts):

select 
      t1.user_id, t1.record_date as last_record_date, max(t1.amount)
from 
      mytable  t1
  inner join
      ( select user_id, max(record_date) as last_record_date
        from mytable
        group by user_id 
      ) t2
    on  t1.user_id = t2.user_id 
    and t1.record_date = t2.last_record_date
group by t1.user_id, t1.record_date