Getting daily counts for events that don't happen every day

Question

I have a customer table in which a new row is inserted when a customer signup occurs.

Problem

I want to know the total numb

Answer 1

You're looking for a way to get all the days listed, even those days that aren't represented in your customer table. This is a notorious pain in the neck in SQL. That's because in its pure form SQL lacks the concept of a contiguous sequence of anything ... cardinal numbers, days, whatever.

So, you need to introduce a table containing a source of contiguous cardinal numbers, or dates, or something, and then LEFT JOIN your existing data to that table.

There are a few ways of doing that. One is to create yourself a calendar table with a row for every day in the present decade or century or whatever, then join to it. (That table won't be very big compared to the capability of a modern database.

Let's say you have that table, and it has a column named date. Then you'd do this.

 SELECT calendar.date AS created,
        ISNULL(a.customer_count, 0) AS customer_count
   FROM calendar
   LEFT JOIN ( 
            SELECT COUNT(*) AS customer_count,
                   DATE(created) AS created
              FROM customer
             GROUP BY DATE(created)
        ) a ON calendar.date = a.created
   WHERE calendar.date BETWEEN start AND finish 
   ORDER BY calendar.date

Notice a couple of things. First, the LEFT JOIN from the calendar table to your data set. If you use an ordinary JOIN the missing data in your data set will suppress the rows from the calendar.

Second, the ISNULL in the toplevel SELECT to turn the missing, null, values from your dataset into zero values.

Now, you ask, where can I get that calendar table? I respectfully suggest you look that up, and ask another question if you can't figure it out.

I wrote a little essay on this, which you can find here.http://www.plumislandmedia.net/mysql/filling-missing-data-sequences-cardinal-integers/