PromQL from Prometheus has steep learning curve because of lack of good documentation. When you understand PromQL basics, then it is much easier to write typical queries over time series data in PromQL than in any other query languages (SQL, Flux, InfluxQL, etc.). I'd recommend reading the article about PromQL basics [1] and then feel the power of PromQL.
[1] https://medium.com/@valyala/promql-tutorial-for-beginners-9a...