Add Extracting Nested JSON Data as a postgres til.

postgres/extracting-nested-json-data.md

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+# Extracting Nested JSON Data
+
+If you are storing nested JSON data in a postgres JSON column, you are
+likely going to find yourself in a situation where you need to access some
+of those nested values in your database code. For instance, you may need to
+get at the license number in this JSON column
+
+```sql
+  owner
+------------------------------------------------------------------------------
+{ "name": "Jason Borne", "license": { "number": "T1234F5G6", "state": "MA" } }
+```
+
+Unfortunately, the `->` operator isn't going to do the trick. You need the
+`json_extract_path` function
+
+```sql
+> select json_extract_path(owner, 'license', 'number') from some_table;
+
+ json_extract_path
+-------------------
+    T1234F5G6
+```
+
+Read more about [JSON Functions and
+Operators](http://www.postgresql.org/docs/9.4/static/functions-json.html).