SQL · Question 14 · Hard
Table A has one column x with rows: 1, 1, 2, NULL Table B has one column x with rows: 1, 1, NULL, 3 What does this query return?
SELECT COUNT(*)
FROM A
JOIN B ON A.x = B.x;- 2
- 5
- 6
- 4
Answer and explanation
Answer: D. 4
Explanation: Each of the two 1s in A matches each of the two 1s in B, giving 2 × 2 = 4 rows. The value 2 has no match, and NULL = NULL is not TRUE, so the NULLs do not join.
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