Another post for me that is simple and hopefully serves as an example for people trying to get blogging as #SQLNewBloggers.
I was editing an article recently that talked about ROLLUP, and I wanted to play with this a bit more. I hadn’t used this much in my career, but it’s a handy operator that’s worth including in your toolkit.
ROLLUP is used to provide additional totals for your aggregates while using GROUP BY. Here’s a little example. I’ve got some data for American Football quaterbacks. In this case, I’ve extracted some stats for a few noteworthy players today. Here’s a sample:
I want to aggregate some data together. For example, let’s say that we want to look at the total touchdowns scored per year by these players in their first 5 years. That’s a simple query:
SELECT
Season = qs.CareerYear
, TDs = SUM(qs.Touchdowns)
FROM dbo.QBStats AS qs
WHERE qs.CareerYear <= 5
GROUP BY qs.CareerYear
This gives me data like this:
However, when I look at this, I don’t have a total across all years. In my set, there are players that have played up to 18 seasons, and I want to know the aggregate number of TDs. I can get that by adding WITH ROLLUP. This is added
after the group by. However, I can also change this to GROUP BY ROLLUP and include the columns in parenthesis.
SELECT
Season = qs.CareerYear
, TDs = SUM(qs.Touchdowns)
FROM dbo.QBStats AS qs
WHERE qs.CareerYear <= 5
GROUP BY ROLLUP (qs.CareerYear)
This is OK, but the NULL isn’t great. What can I do here? I can add an ISNULL or COALESCE to my query and get this:
Useful, but what about if I’m aggregating by individual players? I can add the player to the column list, but I also need to add the player to the ROLLUP (or GROUP BY) list as well. If I do that, I get this:
That’s quite a bit more data, and I now see NULL values in the Player name. What’s this?
These are the totals for that subgroup. In this case, the first NULL above, next to the 1, is the total for all players for season 1. I can use another trick to clean this up:
This now shows some data in an easier to understand fashion, with some totals. We can see for season 1 that 3 players really started their careers in a strong fashion and 3 didn’t.
At the end, however, we get a different result.
This is because we have handled the player name as a NULL for the second column, but not the season as a NULL. For this last row, out “Total” text comes from an ISNULL of the first column. Here we’d want to do this:
This is where COALESCE comes in handy, allowing us to account for multiple NULL values.
ROLLUP is a nice way to get totals for each of your GROUP BY columns. This can make some of your reporting easier, and perhaps even faster by having the totals calculated in the result set.
SQLNewBlogger
This was a fairly easy piece to write. It took longer to get data together and find a set that had two grouping elements and was interesting than write the queries. I also had to play with ROLLUP a bit to clean up the NULLs, which always made this seem like a useless operator.
I think I’d look to use this more if I had the chance for many reports, as summing totals in some client tools is a pain.
