update(sql/07/2): 添加更多常用语句

docs/sql/07/2.md

@@ -92,3 +92,37 @@ from SC
 where between 70 and 79
 ```
 
+## distinct
+distinct 去除重复值
+### 例：查询学生来自哪些城市
+
+```sql
+select distinct address
+from ST
+```
+
+### 查询有人选修的课程的课程数
+
+```sql
+select count(distinct SCid)
+from SC
+```
+
+## count
+
+### 例：查询城市数量
+
+```sql
+select count(distinct address)
+from ST
+```
+
+## AVG（平均值）
+
+### 例：查询特定课程id下的平均值
+
+```sql
+select AVG(chengji)
+from SC
+where SCid = '111100'
+```