Category: T-SQL

I continue a series on window functions:

So far, we’ve seen some really cool things we can do with window functions. That said, there are some limitations with these windows. Some of them are built into how window functions work and others are incomplete implementations of the ANSI SQL standard. Let’s go check it out.

Click through to check it out.

Abayomi Obawomiye has style:

I recently had a requirement to load some data from a source table to another destination table. The destination columns were exactly the same as the source columns with the same data types and length. The only difference was that some columns on the destination table must be encrypted. The task was to use the SHA2_512 encryption algorithm to encrypt the “sensitive” data. I will talk more about the encryption algorithm in another post.

To achieve this, I needed to use the HASHBYTES function in SQL Server. The challenge was that this function used with the SHA2_512 encryption algorithm will return a fixed character length of 64 characters which will be longer than the character length on my destination table. As a result, SQL Server will throw a truncation error. I will demonstrate this below.

One really important point: SHA is not encryption; it’s a hash (which is why the function is HASHBYTES() instead of something like EncryptByKey() as column-level security uses). Hashes are intended to be a one-way trip, whereas encryption implies an ability to decrypt if you have the relevant key details. Here, the use looks to be obfuscating the text of sensitive data fields, perhaps for loading in a dev/test environment, and so the actions themselves are quite reasonable.

By the way, the styles Abayomi talks about are all listed in this Docs page. Turns out that if you’re using a money datatype, you can use CONVERT() to display the end result with commas.

I continue a series on window functions in SQL Server:

As of SQL Server 2019, there is only one ordered set function: STRING_AGG(). I like STRING_AGG() a lot, especially because it means my days of needing to explain the STUFF() + FOR XML PATH trick to concatenate values together in SQL Server are numbered.

STRING_AGG() is interesting in that we categorize it as a window function and yet it violates my first rule of window functions: there isn’t an OVER() clause. Instead, it accepts but does not require a WITHIN GROUP() clause. Let’s see it in action.

Click through for a look at that, as well as a little hint that maybe we’ve seen ordered set functions before in a different guise.

I continue a series on window functions in SQL Server:

CUME_DIST() doesn’t show 0 for the smallest record. The reason for this is in the definition: CUME_DIST() tells us how far along we are in describing the entire set—that is, what percentage of values have we covered so far. This percentage is always greater than 0. By contrast, PERCENT_RANK() forces the lowest value to be 0 and the highest value to be 1.

Another thing to note is ties. There are 117 values for customer 1 in my dataset. Rows 5 and 6 both have a percent rank of 0.0344, which is approximately rank 4 (remembering that we start from 0, not 1). Both rows 5 and 6 have the same rank of 4, and then we move up to a rank of 6. Meanwhile, for cumulative distribution, we see that rows 5 and 6 have a cumulative distribution of 6/117 = 0.5128. In other words, PERCENT_RANK() ties get the lowest possible value, whereas CUME_DIST() ties get the highest possible value.

Click through for much more detail, including examples galore.