Merge pull request #74 from thread/docs-useful-snippets

Create useful-snippets.md with some useful DB queries

docs/useful-snippets.md

@@ -0,0 +1,33 @@
+# Useful snippets
+
+## Database queries
+
+### Query current states for all labels
+```
+SELECT history.label_name, history.created, history.new_state
+FROM history
+JOIN (
+	SELECT label_name, MAX(created) AS max_created
+	FROM history
+	GROUP BY label_name
+) latest_states
+ON
+	history.created = latest_states.max_created AND
+	history.label_name = latest_states.label_name;
+```
+
+### Count number of labels in a specific state
+
+```
+SELECT COUNT(*)
+FROM history
+JOIN (
+	SELECT label_name, MAX(created) AS max_created
+	FROM history
+	GROUP BY label_name
+) latest_states
+ON
+	history.created = latest_states.max_created AND
+	history.label_name = latest_states.label_name
+WHERE history.new_state = 'STATE_NAME';
+```