Why should I consider using an auxiliary numbers table?
In response to a recent newsgroup question, I suggested a solution that would make use of a numbers table. The user responded that it would be a huge waste of space, since they shouldn't have to store every possible integer for this one query.
I disagree. In that individual case (which I will not reproduce here), a query without a numbers table was much more verbose and possibly less efficient.
Besides, there are many other reasons for using a numbers table. These examples will use the following table (which, for my purposes, will not need more than 1,024 rows, but you may need more):
Parsing a string
Let's say you want to accept a comma-delimited list of integers, and you need to grab each value for part of your query. You can use dynamic SQL and an IN clause (but read this article first), you can loop through the list with a function (see Article #2248), or you can use a numbers table to derive the set of numbers from the list. The latter combines coding techniques safer than dynamic SQL and less complex than simulating an array.
You can also do this with a list of strings, though the code is a little more complex. For example:
Finding IDENTITY gaps
While in most circumstances you shouldn't care if your IDENTITY column has gaps, a numbers table can help identify any breaks in the sequence.
Generating date ranges
When you need to generate a set of dates in a range, the typical solution is to create a loop and iterate through the range, adding a day each time. However, a numbers table can help us generate this range as a set, instead of treating each date in the range individually. This way, you can use the code directly in a subquery or in a table-valued function, without having to worry about creating a temporary table to hold the values while you iterate through the loop.
You could extend this example to create your own calendar table (which, like a numbers table, can be very useful—see Article #2519 for more information). The following example will create a calendar table with 1,024 consecutive days, starting at 2004-01-01 (based on the numbers table we created earlier).
Generating time slices
I often see requests for grouping statistics by 5-minute or 15-minute time intervals. A numbers table can help out with this greatly. To keep it distinct from a normal numbers table:
Now, let's assume we're working on a 15-minute breakdown. We're going to populate this table with 96 rows (1-96), since there are 4 15-minute intervals per hour, and 24 hours per day. (If we were using 5-minute intervals, we'd have 288 rows.)
So now, what is the relevant data we're looking to slice up?
One way we can approach this is to use a derived subquery which holds the start and end time of each 15-minute slice, and then join that against our stats table.
This procedure will take any date in the past. You can consider raising an error if the user enters a future date. Or, you can remove this limitation by commenting out the WHERE clause.
If the date used is today's date, it will only generate stats for 15-minute periods that have been completed (including the current, in the odd case where you run the query exactly on a time slice boundary).
Generating IP ranges
Let's say you have an application that requires checking specific IP addresses against a range (say 128.11.0.x). You can use a numbers table to easily output all 256 possible values for the 4th octet, e.g.
For some fun applications of a numbers table, see Vyas' site.
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