Hi hnnhhnnh,

It looks like you're running into two key issues:

1. Joins in DLT – Stream-stream joins require a watermark on the nullable side and a proper time-based condition. Without this, DLT won’t allow the join. Try adding a watermark and ensuring your join condition includes a time range.

2. Window Function Limitation – The first() window function isn’t supported in streaming data unless it's within a time-based window. Instead of using first(), try using aggregation methods like last() to get the most recent non-null value.

A better approach would be to use grouping and aggregation rather than a window function. This way, you can retain the latest s_featureno value where available, without breaking DLT's streaming constraints. If your issue persists, check that your watermarking and event ordering are properly set. Hope this helps!