Programming · Flashcard
10 customers, 4 with orders. How many rows does a LEFT JOIN give?
- ATen, with NULL order columns for the six that have none
- BFour, since only the customers with orders can match
- CSix, returning just the customers who never ordered
- DFourteen, the matches plus every unmatched row of both
Why this is the answer
A LEFT JOIN keeps all ten left-hand rows and fills the right-hand columns with NULL where nothing matched. Four is the inner join, six is what you get by adding WHERE orders.id IS NULL, and fourteen would need a FULL OUTER JOIN with unmatched rows on both sides.
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