fix(guide): simplify directory structure

guide/english/sql/sql-avg-function/index.md

@@ -0,0 +1,25 @@
+---
+title: SQL Avg Function
+---
+## SQL Average (AVG) Function
+
+"Average" is an Aggregate (Group By) Function. It's used to calculate the average of a numeric column from the set of rows returned by a SQL statement.
+
+Here is the syntax for using the function:
+
+```sql
+select groupingField, avg(num_field)
+from table1
+group by groupingField
+```
+
+Here's an example using the student table:
+ 
+```sql
+select studentID, FullName, avg(sat_score) 
+from student 
+group by studentID, FullName;
+```
+
+![image-1](https://github.com/SteveChevalier/guide-images/blob/master/avg_function01.JPG?raw=true)
+