Feat: Add 311 example (#310)

* Create category_by_complaint_source.sql

Add a query snippet

* Create artifact.yaml

* Improve formatting

* Add better comments

Add comments and change variable "num_instances" to "num_compaints" for clarity

datasets/san_francisco_311/docs/queries/artifact.yaml

@@ -0,0 +1,5 @@
+artifact:
+  title: Analyze most prevelant category by complaint source of issue
+  description: In this tutorial we analyze the most likely category corresponding to each complaint source from all 311 reports in San Francisco.
+  vertical: government
+  tier: free

datasets/san_francisco_311/docs/queries/category_by_complaint_source/category_by_complaint_source.sql

@@ -0,0 +1,30 @@
+# What is the most common category for each complaint source?
+
+WITH source_category_counts AS (
+  SELECT
+    source,
+    category,
+    COUNT(1) AS num_complaints
+  FROM
+    `bigquery-public-data`.san_francisco_311.311_service_requests 
+  GROUP BY
+    source, category
+)
+SELECT 
+  source,
+  category, 
+  num_complaints, 
+  num_complaints/total AS fraction_of_source
+FROM
+  (SELECT 
+     source,
+     category, 
+     num_complaints,
+     # Within each source, rank the categories by number of complaints in descending order. 
+     ROW_NUMBER() OVER (PARTITION BY source ORDER BY num_complaints DESC) AS rank,
+     # Compute the total number of complaints reported per source
+     SUM(num_complaints) OVER (PARTITION BY source) total
+   FROM source_category_counts)
+ WHERE
+   # Extract the most common category of complaint
+   rank = 1;