Count rows safely with LEFT JOIN

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Count rows safely with LEFT JOIN

`LEFT JOIN` keeps every row of the left table even when nothing matches, and `COUNT(column)` ignores NULLs — the safe way to count related rows.

Definition and example

Count rows safely with LEFT JOIN is the focus of this lesson. `LEFT JOIN` keeps every row of the left table even when nothing matches, and `COUNT(column)` ignores NULLs — the safe way to count related rows. The task asks you to apply that idea in a small example: Task: Count rows safely with LEFT JOIN. Which rows come back. Before you run the code, predict what should happen. Then Inspect the returned columns and rows and compare it with your prediction. If the result differs, change one relevant line and run it again. Testing one change at a time makes the cause of a result easier to understand.

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-- TASK count-rows-safely-with-left-join-06: Task: Count rows safely with LEFT JOIN. Which rows come back?
-- STEPS: Read the explanation. Change one relevant line at a time, then run the code and compare the result with the check on the right.
-- TODO: Write the query requested in the task comments. Use the sample database shown in Read first.

Run your code to see the output and the checks.

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Checks on this task:

The query must return the expected rows and values.

The task and step-by-step instructions are written as comments at the top of the editor. Change one relevant line, run it, and compare the result with these checks.

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Count rows safely with LEFT JOIN

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