Product Sales Analysis I

Join sales to their product names — one product_id, one name

Report each sale's product_name, year, and price by joining Sales to Product on product_id. (The earlier worked example was inconsistent — it mapped a single product_id to two different names, which is impossible since product_id is the primary key of Product. Here is a consistent trace.)

SELECT p.product_name, s.year, s.price
FROM Sales s
JOIN Product p ON s.product_id = p.product_id;

Traced on consistent data

Product (product_id is PK): (100, Nokia), (200, Apple). Sales:

sale_id	product_id	year	price
1	100	2008	5000
2	100	2009	5000
7	200	2011	9000

Each product_id resolves to exactly one name (100→Nokia every time), so the join is:

product_name	year	price
Nokia	2008	5000
Nokia	2009	5000
Apple	2011	9000

Pitfalls

• Because product_id is the PK of Product, the join is many-sales-to-one-product — a name can never be ambiguous. If your trace shows one id with two names, the trace is wrong.
• Use an INNER JOIN here: a sale always references an existing product (referential integrity). If some sales could reference a missing product and you still want them, that's a LEFT JOIN.

Takeaways

• Join Sales to Product on the PK product_id; one id → one name, always.
• Sanity-check worked examples against the keys — a PK can't map to two values.

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Re-authored for correctness for this guide (the prior trace mapped one product_id to two names). Pattern: LeetCode 1068. See also: INNER JOIN, Keys, Integrity Constraints.

🎯 STRICT STANDOUT: Why / worked / when-not / failure / drills — Product Sales Analysis I

Why this concept exists (judgment chain)

This is the many-to-one PK join pattern: Sales.product_id → Product.product_id (PK) so each sale gets exactly one product_name. Worked examples that show one id mapping to two names are self-contradicting. INNER is correct under referential integrity; LEFT only if orphan sales are possible and wanted.

Worked example with numbers or traced steps

Product: (100,Nokia),(200,Apple) — product_id PK.
Sales: (1,100,2008,5000),(2,100,2009,5000),(7,200,2011,9000).
JOIN → Nokia/2008/5000, Nokia/2009/5000, Apple/2011/9000.
100 → Nokia both times (never two names).
INNER drops sales with missing product; LEFT would NULL-pad name.
Index on Product.product_id (PK) makes probe O(log n) / hash.

When NOT to use / named alternative

Do not LEFT JOIN if FK guarantees every sale has a product and you want to surface integrity violations as missing rows instead of silent NULLs. Do not DISTINCT product_name unless the grain is wrong. Avoid SELECT * when only name/year/price are required.

Failure / ops fingerprint

Trace tables that violate PK uniqueness confuse learners and hide bugs. Orphan product_id in Sales under missing FK → INNER silently drops revenue rows. Ops: FK constraint + orphan-sales monitor; EXPLAIN Nested Loop / Hash Join on the PK.

Hostile-panel Q&As (model answers)

Q1. Why can't one product_id have two names in a correct trace?
Model answer: product_id is PK of Product — uniqueness of the name mapping is a schema invariant.

Q2. INNER vs LEFT here?
Model answer: INNER if RI holds and orphans should vanish; LEFT if you must report sales even when Product row is missing.

Q3. Grain of the result?
Model answer: One row per sale (not per product) — product_name repeats across sales of the same product.

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