Add: PyFlink session window PK fix

Closes #248

Author: github-actions[bot]

_questions/data-engineering-zoomcamp/module-7/030_e08b9c753f_pyflink-sess-window-pk-fix.md

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+---
+id: e08b9c753f
+question: PyFlink session window job fails with 'please declare primary key for sink
+  table when query contains update/delete record' error
+sort_order: 30
+---
+
+Session window aggregations produce updates while the session is still open. The JDBC sink needs a primary key so it knows which row should be updated in the table. Without a primary key, Flink cannot apply the updates and the job fails. Define a primary key in the sink table using the window boundaries and the grouping key (for example window_start, window_end, and PULocationID).