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Recall

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Recall

What does a LEFT JOIN return for a row that has no match in the right table?

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DashboardStep 4 / 12
Recall

What does a LEFT JOIN return for a row that has no match in the right table?

The left row is kept, and every column from the right table comes back as NULL.

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Question 2 of 5

When would you reach for a window function instead of GROUP BY?

When I need a total or a rank next to every row, not one row per group. GROUP BY collapses the rows, a window function keeps them.
Your mentorscored this 4/5

Exactly the right distinction. To make it complete, name how you would split the rows into groups for the calculation.

Worth revisiting: PARTITION BY, frame clauses

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Mastery Map12 concepts
SELECTJoinsGROUP BYWindowsCTEsIndexesSubquery

Window functions

Retention44%
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  • Note

    Joins, in my own words

    LEFT keeps everything on the left. INNER keeps only the matches. Draw the two circles before writing the query.

    SQLjoinsOct 7

  • Note

    Questions for Thursday

    Why does COUNT(*) differ from COUNT(column)? Ask about NULLs in the review.

    to askOct 5
Ready to captureRich note canvas
BodyHBIU
A window function looks at a frame of rows around the current one. PARTITION BY picks the group, ORDER BY sets the direction.
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