Question

Recently, I have been getting familiar with PostgreSQL(using 8.2) and found the date_trunc function extremely useful for easily matching time stamps between certain days/months/etc.
The real usefulness of the function, I believe, comes from the fact that it keeps the output in the format of a timestamp.

I have had to switch to mySQL(5.0) and find some of the date functions rather lacking in comparison. The extract function seems useful and the date function I have found solves some of my problems, but is there any way to replicate PostgreSQL’s date_trunc?

Following is an example of how I used to use date_trunc to match queried timestamps to only the last 4 months including the current month, but only if a week has passed into this month already:

WHERE date_trunc('month', QUERY_DATE) BETWEEN 
    date_trunc('month', now()) - INTERVAL '4 MONTH' AND 
    date_trunc('month', now() - INTERVAL '1 WEEK')

I have no idea how to recreate such a stipulation in mySQL. So, my question at the end of the day, is whether this type of query can be accomplished in mySQL by trying replicate date_trunc(and how) or whether I need to start looking at these types of queries in a different way to make them work in mySQL(and suggestions on how to do that)?

Answer 1

The extract function seems useful and the date function I have found solves some of my problems, but is there any way to replicate PostgreSQL’s date_trunc?

Indeed, EXTRACT looks like it’s going to be the closest match for this specific case.

Your original code in PG:

WHERE date_trunc('month', QUERY_DATE) BETWEEN 
    date_trunc('month', now()) - INTERVAL '4 MONTH' AND 
    date_trunc('month', now() - INTERVAL '1 WEEK')

Using EXTRACT:

WHERE EXTRACT(YEAR_MONTH FROM QUERY_DATE)
      BETWEEN
          EXTRACT(YEAR_MONTH FROM NOW() - INTERVAL 4 MONTH)
      AND
          EXTRACT(YEAR_MONTH FROM NOW() - INTERVAL 1 WEEK)

While it should be functionally identical, this is actually mangling the dates into a YYYYMM string before doing the comparison.

Another option would be using DATE_FORMAT to rebuild the date string and force it to the beginning of the month:

WHERE DATE_FORMAT(QUERY_DATE, '%Y-%m-01')
      BETWEEN
          DATE_FORMAT(NOW() - INTERVAL 4 MONTH, '%Y-%m-01')
      AND
          DATE_FORMAT(NOW() - INTERVAL 1 WEEK, '%Y-%m-01')

Also, be aware that MySQL is really poor at dealing with date ranges, even when the field is indexed. You’re probably going to end up with a full table scan if you aren’t careful.