select distinct student_id
from student_course
group by student_id
having count(*) >= 2;
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SELECT * FROM student WHERE MONTH(birthday) = MONTH(DATE_ADD(NOW(), INTERVAL 1 MONTH));
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UNION：求两个或多个集合的并集，去除重复
UNION ALL：求两个或多个集合的并集，不去除重复
INTERSECT：求两个或多个集合的交集，去除重复
MINUS：求两个或多个集合的差集，去除重复
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```
SELECT * FROM student WHERE student_id IN (SELECT student_id FROM elective GROUP BY student_id HAVING CO
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1、使用MySQL命令行客户端：
可以使用source命令实现，将多个.sql文件导入到一个MySQL数据库中：
```
mysql> source /path/to/file1.sql;
mysql> sour
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秒级时间戳：
```
SELECT UNIX_TIMESTAMP();
```
毫秒级时间戳：
```
SELECT UNIX_TIMESTAMP(NOW()) * 1000;
```
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