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What it does: For every order placed since January 1, 2026, this query shows the customer, date and amount, plus two extra columns: how much that customer has spent so far (a running total) and how recent the order is compared with that customer's other orders. Step by step: 1. FROM orders WHERE order_date >= '2026-01-01' takes only this year's orders. 2. SUM(amount) OVER (PARTITION BY customer_id ORDER BY order_date) adds up amounts one customer at a time, in date order. Each row shows the total up to and including that order, so a customer's third order shows the sum of their first three. 3. ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY order_date DESC) numbers each customer's orders from newest (1) to oldest. Filtering on recency_rank = 1 in an outer query would give each customer's latest order. 4. Unlike GROUP BY, these window functions keep every row; they add columns instead of collapsing rows. Things to know: - If a customer has two orders on the same date, the running total counts both on each of those rows, because the default window includes ties. Add a tie-breaker such as order_id to ORDER BY if you want one row at a time. - Orders before 2026 are excluded before the running total is calculated, so it's a total for this year, not all time. - You can't use recency_rank in this query's WHERE clause; wrap the query and filter outside it.

What it does: Finds every file ending in .log under the current folder that was last modified more than 7 days ago, and compresses each one with gzip. Step by step: 1. find . searches the current folder and every folder inside it. 2. -name '*.log' keeps files whose names end in .log. The quotes stop the shell from expanding *.log before find sees it. 3. -mtime +7 keeps files modified more than 7 full days ago. 4. -print0 outputs the file names separated by a null character instead of a new line, so names with spaces don't break. 5. xargs -0 reads those null-separated names and passes them to gzip in batches. 6. gzip compresses each file in place: app.log becomes app.log.gz and the original is removed. Things to know: - It also matches folders named something.log. Add -type f after find . to match files only. - If a program still writes to one of these logs, compressing it can lose new lines. Stop the program or use your log rotation tool. - Run it first with -print instead of -print0 | xargs -0 gzip to see which files it will touch.

What it does: When this component first appears on screen, it starts a timer that adds 1 to count every second, and it stops the timer when the component is removed. Step by step: 1. useEffect(() => { ... }, []) runs the code inside once, after the component first renders. The empty array means "don't run this again on later renders". 2. setInterval(..., 1000) calls a function every 1,000 milliseconds and returns an id for that timer. 3. setCount(c => c + 1) updates count using the latest value. This matters: because the effect only runs once, writing setCount(count + 1) would always use the count from the first render and get stuck at 1. 4. return () => clearInterval(id) is the cleanup. React runs it when the component unmounts, so the timer doesn't keep running in the background. Things to know: - In development with React Strict Mode, React mounts, cleans up and remounts once on purpose, so you may see the effect run twice. The cleanup is what keeps that from creating two timers. - Browsers slow down timers in background tabs, so count can fall behind real seconds. Store a start time and compute elapsed time if accuracy matters.