SQLMaxx
Sign in Start free
N003-H1 Tier 1 · Foundations · hard ecommerce · Brightlane

Return the name and total inventory value for every flagged product

Part of WHERE Clause and Comparison Operators in SQL

The problem

Brightlane's warehouse manager flags products for a manual stock audit when their total inventory value exceeds $5,000.

Write a query to return the name and total inventory value for every flagged product.

Assumptions:

  • The products table contains every item in Brightlane's catalog.
  • The price column is each product's unit price; the stock_qty column is the number of units currently in stock.
  • Inventory value is price * stock_qty.

Output:

  • One row per flagged product, with columns name and inventory_value.
Schema · ecommerce 5 tables
categories
id integer
name text
parent_id? integer
products
id integer
name text
category_id integer
price numeric
stock_qty integer
attributes? jsonb
order_items
id integer
order_id integer
product_id integer
quantity integer
unit_price numeric
customers
id integer
name text
email text
city? text
country text
created_at timestamptz
is_active boolean
orders
id integer
customer_id integer
ordered_at timestamptz
status text
total_amount numeric
Check answerShift Ctrl ↵

Run previews · Check grades

Write a query, then run it to see results here.
Worked solution Try it yourself first
Solution query
SELECT
  name,
  price * stock_qty AS inventory_value
FROM
  products
WHERE
  price * stock_qty > 5000

The shape

The inventory_value column doesn't exist in the products table — it's price * stock_qty. So the same calculation has to appear in two places: in the SELECT list to produce the column, and inside WHERE to filter on its value.

Clause by clause

  • SELECT name, price * stock_qty AS inventory_value returns the product name and the computed total inventory value. The multiplication runs once per surviving row, and the AS alias gives the column the name the audit report expects.
  • FROM products is the source — every item in Brightlane's catalog with its price and current stock.
  • WHERE price * stock_qty > 5000 is the audit threshold. PostgreSQL evaluates price * stock_qty for each row, compares the result to 5000, and keeps the row when the product clears the threshold. Strict > excludes the boundary, matching the prompt's "exceeds $5,000."

Why the calculation is repeated in WHERE

The alias inventory_value doesn't exist at the time WHERE runs. The evaluation order is FROM first, then WHERE, then SELECT. The alias only comes into being once SELECT finishes producing its output columns, and by then the filtering is over. Writing WHERE inventory_value > 5000 would raise an error: the name isn't defined yet.

So the calculation has to be written out again inside WHERE. PostgreSQL doesn't notice that the same expression appears twice — it evaluates each one independently against the row.

The trap

The two copies of price * stock_qty have to stay in sync. If the SELECT expression were price * stock_qty and the WHERE expression were price * units (or vice versa), the query would either error or — worse — quietly return rows that don't match what the output column actually shows. Whenever a derived value appears in both SELECT and WHERE, the two expressions must be character-for-character identical in the columns and operators they reference. The audit list is only trustworthy when the filter and the displayed value compute the same number.

You practiced filtering on a value the source table doesn't expose directly — the inventory value has to be computed from two columns and tested against the threshold. The derived-then-filtered shape recurs anywhere a condition applies to a calculation.

How you actually get good at SQL

Reading explains SQL. Writing it, over and over with instant feedback, is what makes you fluent.

That's the whole SQLMaxx loop: 600+ real problems, instant AI feedback, mastery you can actually see, and spaced review that won't let you forget.

A stack of SQL practice problem cards, the top card showing an employees table.
615 problems · 66 concepts

Real problems. Not toy examples.

615 hand-built problems spanning all 66 concepts, from basic SELECTs to window functions, built on real schemas and real business questions, the kind you'll actually get asked on the job. Enough reps to make SQL automatic.

A retro computer showing a SQL query marked correct with a green checkmark.
Instant AI feedback

Write a query. Know if it's right in one second.

No copying an answer and hoping it clicked. The AI grader checks your real query against real data, catches exactly what's wrong, and explains the fix in plain English, like a senior analyst reading over your shoulder on every problem.

A circular mastery progress dial filling from blue to green, the SQLMaxx diamond at its center.
Mastery tracking

Stop guessing whether you actually know it.

SQLMaxx tracks every concept and shows you what you've mastered and what's still shaky. Your skills fill in one concept at a time, so 'I think I get joins' becomes something you can prove.

A SQL query editor circled by a blue return arrow with a clock, scheduled to come back for review.
Spaced review

Learn it once. Keep it for good.

Most of what you learn this week fades by next week. So when a concept comes due for review, SQLMaxx hands you a fresh problem to solve from a blank editor, not a flashcard to re-read. A research-backed spaced-repetition algorithm (FSRS) times each return for right before you'd forget, so your SQL is still there months later, when the interview or the job actually needs it.

Practice, feedback, mastery, review. That's the loop that turns reading into real skill.

Start free

No account, no credit card. Start solving in under a minute.