Question

I have a table with records that look like this:

CREATE TABLE sample (
  ix int unsigned auto_increment primary key,
  start_active datetime,
  last_active datetime
);

I need to know how many records were active on each of the last 30 days. The days should also be sorted incrementing so they are returned oldest to newest.

I’m using MySQL and the query will be run from PHP but I don’t really need the PHP code, just the query.

Here’s my start:

SELECT COUNT(1) cnt, DATE(?each of last 30 days?) adate
FROM sample
WHERE adate BETWEEN start_active AND last_active
GROUP BY adate;

Answer 1

Do an outer join.

No table? Make a table. I always keep a dummy table around just for this.

create table artificial_range( 
  id int not null primary key auto_increment, 
  name varchar( 20 ) null ) ;

-- or whatever your database requires for an auto increment column

insert into artificial_range( name ) values ( null )
-- create one row.

insert into artificial_range( name ) select name from artificial_range;
-- you now have two rows

insert into artificial_range( name ) select name from artificial_range;
-- you now have four rows

insert into artificial_range( name ) select name from artificial_range;
-- you now have eight rows

--etc.

insert into artificial_range( name ) select name from artificial_range;
-- you now have 1024 rows, with ids 1-1024

Now make it convenient to use, and limit it to 30 days, with a view:

Edit: JR Lawhorne notes:

You need to change “date_add” to “date_sub” to get the previous 30 days in the created view.

Thanks JR!

create view each_of_the_last_30_days as
select date_sub( now(), interval (id - 1) day ) as adate
from artificial_range where id < 32;

Now use this in your query (I haven’t actually tested your query, I’m just assuming it works correctly):

Edit: I should be joining the other way:

SELECT COUNT(*) cnt, b.adate
FROM  each_of_the_last_30_days b
left outer join sample a 
 on ( b.adate BETWEEN a.start_active AND a.last_active)
GROUP BY b.adate;