FA-124 / Storage and queries / Member archive

A descriptive join multiplies an invoice subtotal · case 04

Adding multiple descriptive tags inflates the subtotal by repeating each invoice line in a join product.

Member previewVariant 4 · 3 implementations · 7 checks per implementation

Case contract

Given one invoice's integer line amounts and descriptive tag strings, return [sum_of_all_lines, sorted_unique_tags]. Each line occurrence contributes once, even equal-priced lines. Missing tags do not remove an invoice or its revenue.

Why this case matters

Models a many-to-many query fanout and grain mismatch, a frequent analytical-query error. It makes the relational cardinality explicit without claiming to reproduce a full billing database.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
two lines crossed with multiple tags	`[18, ["blue", "red"]]`	`[9, ["blue", "red"]]`	Failed

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