Return the name and price-per-unit-in-stock of every qualifying product

The shape

NULLIF(stock_qty, 0) makes the division safe in both the SELECT list and the WHERE clause, and NULL's behavior in a comparison handles the rest. A sold-out product yields a NULL price / NULLIF(stock_qty, 0), and NULL > 50 is not true, so the row drops out of the filter on its own. No separate stock_qty <> 0 check is needed.

Clause by clause

• SELECT name, price / NULLIF(stock_qty, 0) AS price_per_unit returns the name and per-unit price for each row that survives the filter. The NULLIF swaps a zero stock value for NULL before the division, so the expression yields a real number for in-stock products and NULL for sold-out ones.
• FROM products reads the catalog. Sold-out products will be filtered out by the WHERE clause downstream.
• WHERE price / NULLIF(stock_qty, 0) > 50 re-evaluates the same guarded division and keeps only the rows where the result exceeds 50. For a sold-out product, the division returns NULL, and NULL > 50 returns NULL, which WHERE treats as not-true. Those rows are silently dropped, which is exactly the behavior the report needs.

Why repeat the expression in WHERE and not just WHERE price / stock_qty > 50

The unguarded division in the WHERE would raise a division-by-zero error the moment PostgreSQL reaches a sold-out product, and the whole query would fail. Wrapping stock_qty in NULLIF(stock_qty, 0) in both places keeps the divisor non-zero everywhere it appears. The duplication reads as repetition but is load-bearing: each occurrence of the division needs its own guard.

The trap

The filter looks like it might also keep sold-out rows with price_per_unit showing as NULL, since the SELECT list produces NULL for them. It does not. WHERE keeps a row only when its condition evaluates to true; NULL is not true. A WHERE predicate that returns NULL drops the row exactly as if it had returned false. This is the same NULL-vs-three-valued-logic rule that governs every WHERE comparison, and knowing it means an explicit WHERE stock_qty <> 0 is unnecessary because the NULL produced by NULLIF already does that work.