Calculating Skew In SQL

Lukas Eder shows how you can use PERCENTILE_DISC to calculate skewness in SQL:

In RDBMS, we sometimes use the term skew colloquially to mean the same thing as non-uniform distribution, i.e. a normal distribution would also be skewed. We simply mean that some values appear more often than others. Thus, I will put the term “skew” in double quotes in this article. While your RDBMS’s statistics contain this information once they are calculated, we can also detect such “skew” manually in ad-hoc queries using percentiles, which are defined in the SQL standard and supported in a variety of databases, as ordinary aggregate functions, including:
– Oracle
– PostgreSQL
– SQL Server (regrettably, only as window functions)

As Lukas implies, SQL Server is a step behind in terms of calculating percentiles, and calculating several percentiles over a large data set will be slow. Very slow. Though batch mode processing in 2019 does help here.

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