34. Customers Who Placed Orders
MediumJoinssemi-joinEXISTSIN
A semi-join returns rows from one table that have at least one match in another — but without duplicating them per match. We want each qualifying customer once, no matter how many orders they placed.
Task: return the name of every customer who has placed at least one order. A plain INNER JOIN would list Ava twice (she has two orders); a semi-join lists her once.
Two idiomatic forms are shown: EXISTS and IN.
Output column: name. Order by name ascending.
Tables
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Expected outputrows must come back in this order
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