MySQL: Count of records with consecutive months

Question

I\'ve searched around for this, but all the similar questions and answers are just different enough not to work.

I have a table with the following fields: person, th

Answer 1

You can do this in MySQL using variables (or very complicated correlated subqueries). In other databases, you would use window/analytic functions.

The logic is:

1. Get one row per month and person with a purchase.
2. Use variables to assign each group of consecutive months a "grouping" value.
3. Aggregate by the person and the "grouping" value.

Here is a query that has been tested on your SQL Fiddle:

select person, count(*) as numMonths
from (select person, ym, @ym, @person,
             if(@person = person and @ym = ym - 1, @grp, @grp := @grp + 1) as grp,
             @person := person,
             @ym := ym
      from (select distinct person, year(purdate)*12+month(purdate) as ym
            from records r
           ) r cross join
           (select @person := '', @ym := 0, @grp := 0) const
      order by 1, 2
     ) pym
group by person, grp;