I don't have anything better to point you at than the help text, but I can try to give some basic information that is relevant to your example.
First, you'll want to understand HOW set analysis does the comparison. In this case, you're trying to "select" a specific month. You do this by telling it which month to select. But it isn't like a normal =, a normal date comparison. Instead, set analysis behaves as if what you were typing was going in a search expression in a list box. One thing to take from that is that it will always therefore be a string sort of comparison, not a numeric or date sort of comparison.
In other words, if "month" is defined as the first day of the month, and formatted as 'MMM YYYY', so it shows up in your list box like 'Jun 2010', you somehow have to end up with that TEXT inside of the set analysis expression to match it.
Something else you'll want to know that will put you ahead of a lot of people is the difference between using single quotes and double quotes.
month={Jun 2010} <-- this won't work
month={'Jun 2010'} <-- this is the LITERAL 'Jun 2010'
month={"Jun 2010"} <-- this is the SEARCH EXPRESSION "Jun 2010"
What's the difference? Well, not much when you're just specifying a single value like this. Searching for Jun 2010 will give the same result as just selecting the literal Jun 2010. But I believe you MUST use literals if you're specifying multiple values:
month={'Jun 2010','Jul 2010'}
And you MUST use a search expression when you're searching for a range:
month={'<=Jun 2010'} <-- this won't work; it's looking for '<= Jun 2010' as a literal value for month
month={"<=Jun 2010"} <-- yes, it's strange, but this works even with the space because it's a search expression
So best to just get in the habit of using the right kind of quotes for the right situation.
Now let's apply what we've learned. We want to use max(month) somehow. But we're going to need to do that as our very first step, before we start messing around with restricting our month value. We also can see that we need to literally insert our month into the expression, because "max(month)" isn't a valid search expression, and 'max(month)' certainly isn't the literal we're looking for. Fortunately, a dollar sign expansion covers all those bases. So as a first pass, we're looking for something like this:
$(=max(month))
Now, what happens if you use that expression? Well, max(month) returns a date field, but fortunately, it's a formatted Date field, and as best I can tell, it preserves the format of the "month" field. If it didn't, and not all expressions work so nicely, we'd need to do this to format it manually, so that's often a good first thing to try if you're having trouble:
$(=date(max(month),'MMM YYYY')))
The dollar sign expansion actually literally returns your maximum month, Jun 2010 for the sake of argument. NOT a date or a month. Fortunately, that's just what we need since set analysis can't do a date or numeric comparison.
However, it has a space in it. That won't work, because QlikView won't realize it's a literal. We have to enclose it in single quotes to tell QlikView that it is a literal:
'$(=max(month))'
That should LITERALLY return this when executed:
'Jun 2010'
Which we've already seen we can use in set analysis. So now we just plug our expression so far where we had 'Jun 2010' in the example much further up:
month={'$(=max(month))'}
I would presume you want that to be the maximum of the months SELECTED. If you want the maximum of ALL months, you'll need to use set analysis INSIDE of the max(month) expression:
month={'$(=max({1}month))'}
I'm also assuming that the ONLY thing we're trying to override is the month. If you've selected five clients, you only want to count from THOSE clients, not from all clients. The set expression to do that is this:
{$<month={'$(=max(month))'}>}
The dollar sign here is NOT dollar sign expansion, but a way of telling set analysis that you're modifying the currently-selected set of data, not starting over. However, that's also the default, so you can leave the $ off. I always leave it off, in the hopes of avoiding confusion with dollar sign expansion.
{<month={'$(=max(month))'}>}
So we're almost there. Now you just have to add the count(distinct client):
count({<month={'$(=max(month))'}>} distinct client)
And of course do your desired subtraction:
count({<month={'$(=max(month))'}>} distinct client)
- count({<month={'$(=min(month))'}>} distinct client)
With any luck, that should do it.
