Introduction to SQL
Which of the following query finds the name of the sailors who have reserved at least two boats?

All of these
SELECT DISTINCT s.sname FROM sailors s, reserves r1, reserves r2 WHERE s.sid = r1.sid AND COUNT(r1.bid) > r2.bid
SELECT DISTINCT s.sname FROM sailors s, reserves r1, reserves r2 WHERE s.sid = r1.sid AND r1.sid = r2.sid AND r1.bid <> r2.bid
SELECT DISTINCT s.sname FROM sailors s, reserves r1, reserves r2 WHERE s.sid = r1.sid AND r1.sid = r2.sid AND r1.bid ≠ r2.bid

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Introduction to SQL
NULL is

the same as 0 for integer and blank for character
the same as 0 for integer
not a value
the same as blank for character

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