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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.
w3schools is one starting point for basic SQL. Some college instructors refer their students to this.
You can use this Adventureworks for Postgres repo to practice.
NOTE:
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Hence, a higher number means a more popular project.