sql经典实例
### 一、学生成绩数据统计
查询班级全体学生成绩平均分：
```sql
SELECT AVG(score) FROM student WHERE class_id = 'S001'
```
查询各科学生平均成绩：
```sql
SELECT subject, AVG(score) FROM student GROUP BY subject
```
查询某班级某科大于90分人数：
```sql
SELECT COUNT(*) FROM student WHERE class_id = 'S001' AND subject = 'Math' AND score > 90
```
查询某科不及格学生总数：
```sql
SELECT COUNT(*) FROM student WHERE subject = 'Math' AND score < 60
```
### 二、用户信息查询
sql存储过程实例查询年龄大于30岁的用户：
```sql
SELECT * FROM user WHERE age > 30
```
查询某个地区(城市)的用户：
```sql
SELECT * FROM user WHERE city = 'Beijing'
```
查询某个时间段注册的用户：
```sql
SELECT * FROM user WHERE register_date BETWEEN '2016-01-01' and '2016-12-31'
```
查询某个时间段活跃的用户：
```sql
SELECT * FROM user WHERE last_login_time BETWEEN '2017-01-01' and '2017-12-31'
```

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