Lesson 20 · Build and change data

Preview before changing rows

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INSERT adds rows, UPDATE changes values, and DELETE removes rows. Before an UPDATE or DELETE, use a SELECT with the same WHERE condition to inspect its intended scope.

A missing WHERE can affect every row. A syntactically valid statement can therefore be a serious reasoning error. State what should remain unchanged and plan a query to check it.

This preview activity is read-only. The row-change workshops linked below use disposable exercise fixtures and check the resulting data, so you can safely practice the actual statements there.

Worked example

SELECT id, name, price FROM menu_items WHERE price < 4 ORDER BY id;

This SELECT previews the rows below four dollars. It cannot change any prices, so it is a useful first check before an UPDATE.

For the practice task, add both category and price restrictions. The verification plan should include qualifying rows and a sample or full comparison of rows that must remain unchanged.

Your task

Return id, name, and price for Tea items priced below 5, ordered by id. This is the scope preview for a proposed price change.

Predict or plan before trying

Give me a hint

Preview exactly the rows an eventual UPDATE would target.

Compare with a worked answer

SELECT id, name, price FROM menu_items WHERE category = 'Tea' AND price < 5 ORDER BY id;

Using an answer is useful practice. Record what you learned; it does not demonstrate independent mastery.

Table & column names only · ↑ ↓ choose · Tab insert · Esc dismiss

Query results

Run your query to see the result here.

Explain your evidence

Which unchanged rows would you inspect afterward, and why is checking only the changed count insufficient?

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