Document group by example for Soda Core with failed rows check (#1984)

* Added group by example for Soda Core with failed rows check

* Adjusted SQL per suggestion

* Corrected SQL

examples/group-by.md

@@ -0,0 +1,39 @@
+# Group check results by category with Soda Core
+
+You can use a SQL query in a failed row check to group failed check results by one or more categories using Soda Core. 
+
+Use a SQL editor to build and test a SQL query with your data source, then add the query to a failed rows check to execute it during a Soda scan.
+
+The following example illustrates how to build a query that identifies the countries where the average age of people is less than 25.
+
+1. Begining with a basic query, the output shows the data this example works with.
+```sql
+SELECT * FROM Customers;
+```
+![group-by-1](/docs/assets/images/group-by-1.png){:height="600px" width="600px"}
+2. Build a query to select groups with the relevant aggregations.
+```sql
+SELECT country, AVG(age) as avg_age
+FROM Customers
+GROUP BY country
+```
+![group-by-2](/docs/assets/images/group-by-2.png){:height="600px" width="600px"}
+3. Identify the "bad" group (where the average age is less than 25) from among the grouped results.
+```sql
+	SELECT country, AVG(age) as avg_age
+	    FROM Customers
+	    GROUP BY country
+    	    HAVING AVG(age) < 25
+```
+![group-by-3](/docs/assets/images/group-by-3.png){:height="600px" width="600px"}
+4. Now that the query yields the expected results, add the query to a failed row check, as per the following example.
+```yaml
+checks for dim_customers:
+  - failed rows:
+      name: Average age of citizens is less than 25
+      fail query: |
+	    SELECT country, AVG(age) as avg_age
+	        FROM Customers
+	        GROUP BY country
+          	HAVING AVG(age) < 25
+```

examples/postgres_example.md

@@ -71,21 +71,15 @@ checks for dim_customer:
        name: No duplicate phone numbers
   - freshness(date_first_purchase) < 7d:
        name: Data in this dataset is less than 7 days old
-  - schema:
-       warn:
-         when schema changes: any
-       name: Columns have not been added, removed, or changed
+
 EOT
 
-# run the scan!
+# run the scan
 soda scan -d adventureworks -c configuration.yml checks.yml
-
-# note that an error is thrown for one test, as change-over-time checks
-# require you to connect to Soda Cloud
-
 ```
 
 
 
 
 
+