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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
They also have a GitHub repo with said challenges.
Do Kaggle competitions count? Also, there's Project Euler and LeetCode for collections of small coding challenges: the former is more oriented towards math, and the latter is more general-purpose (although it does have a Machine Learning 101 section -- I can't vouch for it, I've never used it myself), but they could maybe help with general coding skill.
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