**Why It Matters (Architectural Logic)**: Multi-source joins require consistent keys, null handling, and skew mitigation. Netflix-scale = partition by business dims, broadcast small tables.
Join and clean multiple sources with consistent keys and null handling:
```python
df1_clean = df1.dropDuplicates(["id"]).na.fill({"region": "Unknown"})
df2_clean = df2.filter(F.col("amount") > 0).dropDuplicates(["id"])
joined = df1_clean.join(df2_clean, "id", "left").drop(df2_clean.id)
final = joined.withCol...
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