Question

I have a table with entries which has a DATE field. Each entry has a distinct date.
I’d like to select all the entries from the last month registred in the database. How?

I tried:

SELECT * 
  FROM registries 
 WHERE reg_date = DATE_FORMAT(MAX(reg_date), "%m")` 

…without success

Answer 1

Based on OMG Ponies’ query with corrections:

SELECT 
  r.*
FROM 
  registries AS r
JOIN (
  SELECT 
    MAX(t.reg_date) AS max_date
  FROM 
    registries AS t) AS t 
ON DATE_FORMAT(t.max_date, '%Y-%m') = DATE_FORMAT(r.reg_date, '%Y-%m')

Though the performance of the query wouldn’t be excellent, since it would operate the JOIN on two calculated values.
I believe it can still perform decently unless you start hitting millions of records.

On the other hand, you could probably run it faster by querying first for the MAX(reg_date)

SELECT 
  CONCAT(DATE_FORMAT(MAX(r.reg_date), "%Y-%m"), '-01') AS first_day
FROM 
  registries AS r

And then injecting the result in a query:

SELECT 
  r.*
FROM 
  registries AS r
WHERE
  r.reg_date BETWEEN '<first_day>' AND LAST_DAY('<first_day>')

With first_day as a place holder for the previous’ query result.
Provided you indexed reg_date, this should run pretty fast.

Note: LAST_DAY is a MySQL only function.