问题596：有一个courses表 ，有student(学生)和class(课程)。请列出所有超过或等于5名学生的课。学生在每个课中不应被重复计算。例如，表：
应该输出：
解法一：
SELECT DISTINCT tab1.class
FROM courses AS tab1
WHERE (SELECT COUNT(DISTINCT tab2.student)
FROM courses AS tab2
WHERE tab2.class = tab1.class) >= 5
运行结果：
解法二：
SELECT DISTINCT tab1.class
FROM courses AS tab1
WHERE (SELECT COUNT(tab2.student)
FROM (SELECT DISTINCT * FROM courses) AS tab2
WHERE tab2.class = tab1.class) >= 5
运行结果：
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