How to Emulate PERCENTILE_DISC in MySQL and Other RDBMS

In my previous article, I showed what the very useful percentile functions (also known as inverse distribution functions) can be used for. Unfortunately, these functions are not ubiquitously available in SQL dialects. As of jOOQ 3.11, they are known to work in these dialects: DialectAs aggregate functionAs window function MariaDB 10.3.3NoYes Oracle 18cYesYes PostgreSQL 11YesNo … Continue reading How to Emulate PERCENTILE_DISC in MySQL and Other RDBMS

Calculate Percentiles to Learn About Data Set Skew in SQL

B-Tree indexes are perfect when your data is uniformly distributed. They are not really useful, when you have skewed data. I'll explain later why this is the case, but let's first learn how to detect "skew" What is skew? Skew is a term from statistics when a normal distribution is not symmetric. The example given … Continue reading Calculate Percentiles to Learn About Data Set Skew in SQL