Question

Take this simple query:


SELECT DATE_FORMAT(someDate, '%y-%m-%d') as formattedDay
FROM someTable
GROUP BY formatterDay


This will select rows from a table with only 1 row per date.
How do I ensure that the row selected per date is the earliest for that date, without doing an ordered subquery in the FROM?

Cheers

Answer 1

If you are using MySQL there is no way to do what you want in a single select. You would need an inner query that selects the min ID and formatted or other field for each formatted date. Then you join it on the parent table again and select records with a minimum ID.

In other databases (postgresql, oracle, db2) there are windowing functions that allow to do this in a single select (i.e. DENSE_RANK() OVER …).

Here is an example with an inner query (which you may already be aware of):

SELECT B.formattedDay, A.*
  FROM someTable A
       JOIN 
       (
           SELECT DATE_FORMAT(someDate, '%y-%m-%d') as formattedDay,
                  MIN(ID) as firstId
             FROM someTable I
         GROUP BY 1
       ) B ON (A.ID = B.firstId)

Change MIN(ID) for MIN(someDate) if needed and its a timestamp. If more than one record can appear with the same timestamp you will need to perform the operation twice. First getting minimum date per day, then minimum ID per minimum date.