This took me longer than I'd like to admit to structure properly.
Start by explaining the general pattern: use boolean aggregation (e.g., BOOL_OR) within a GROUP BY to create flags per group, then chain those flags in subsequent sub-queries to progressively filter and compute the final metric. Emphasize why a single GROUP BY pass fails: because each stage's filter depends on the aggregated result of the previous stage, requiring sequential evaluation.
Pro tip: Mention that while multiple passes are logically necessary, you can sometimes optimize by using window functions or CTEs to avoid redundant scans, but clarity and correctness should come first in an interview.
Restate the problem to ensure you understand each stage's filter condition and how they depend on previous results. Confirm that the final metric requires chaining flags across stages.
Describe how BOOL_OR (or MAX for booleans) within a GROUP BY can create a flag per group indicating whether any row in the group meets a condition. This flag becomes the input for the next stage.
Show how to use sub-queries or CTEs to pass the flags from one stage to the next, applying additional filters and aggregations at each step. Emphasize that each stage operates on the aggregated output of the previous stage.
Highlight that a single GROUP BY cannot handle sequential dependencies because the filter at stage N depends on the aggregated result of stage N-1, which isn't available until after the GROUP BY. Thus, multiple passes are required.
Mention potential performance considerations and alternatives like window functions or recursive CTEs, but stress that the multi-pass approach is conceptually clear and often necessary.
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