-- 先取并集,然后查询student2是否两条（根据id排序）
SELECT student FROM (SELECT student FROM A union SELECT student FROM B) a1,A a2 WHERE a2.student=a1.student and (SELECT count(*) FROM B GROUP BY student) >= 2;
-- 或者
SELECT * FROM A a, B b WHERE a.student=b.student ORDER BY b.student HAVING COUNT(*)>=2;
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