Questions / Quant development, SQL & data pipelines
Know a join’s cardinality
A join key appears twice in table A and three times in table B. An inner equijoin has no further condition. How many joined rows does that key produce?
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Answer: 6 rows
- Fix one A row: it matches all three B rows carrying the key.
- There are two such A rows, so total matching pairs are 2 × 3=6.
- An equijoin does not select just one matching row or remove duplicates unless the query says to do so.
Interview shortcut. Inner join rows per key = left count × right count.
Takeaway. The data relationship determines join cardinality.
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