Views / Materialized views

Author: clarklindev

01.SQL.md

@@ -91,6 +91,8 @@
 
 [14. Views](#14-views)
 
+[15. Materialized Views](#15-materialized-views)
+
 ---
 ###### <div style="text-align:right">[table of contents](#table-of-contents)</div>
 ## 1. Basics
@@ -324,14 +326,18 @@ No performance benefits between types
 Can provide string in almost any format and postgres will do conversion
 can specify something to be date by explicitly giving a data type with '::DATE'
 
+###### date_trunc('week')
+pulls out from date property and rounds down to nearest week
+
 ```SQL
 SELECT('NOV 20 1980'::DATE);
 ```
   * 1980-11-20 -> 1980-11-20
   * NOV-20-1980 -> 1980-11-20
   * 20-Nov-1980 -> 1980-11-20
   * 1980-November-20 -> 1980-11-20
-  * November 20, 1980 -> 1980-11-20
+  * November 20, 1980 -> 1980-11-20\
+
 
 ##### Time
 
@@ -1502,3 +1508,41 @@ syntax: CREATE OR REPLACE VIEW _ AS
 ```SQL
 	DROP VIEW recent_posts;
 ```
+---
+## 15. Materialized views
+Query that gets executed only at very specific times, but the results are saved and
+can be referenced without rerunning the query.
+
+We make use of a materialized view when we have a very expensive query.
+We can run a materialized view just one time and hang on to the results and refer back to it 
+without having to rerun the very expensive query.
+'WITH DATA' is when we create the materialized view, tell postgreSQL to run query once and hold onto the results.
+after running the materialized view, you can refer to it without running the query again.
+
+
+```SQL
+CREATE MATERIALIZED VIEW as weekly_likes AS (
+  --query to materialize
+  SELECT 
+    data_trunk('week', COALESCE(posts.created_at, comments.created_at)) AS week
+    COUNT(posts.id) AS num_likes_for_posts,
+    COUNT(comments.id) AS num_likes_for_comments
+  FROM likes
+  LEFT JOIN posts ON posts.id = likes.post_id
+  LEFT JOIN comments ON comments.id = likes.comment_id
+  GROUP BY week
+  ORDER BY week
+) WITH DATA; --'with data' is when we create the materialized view, tell postgreSQL to run query once and hold onto the results.
+
+```
+
+```SQL
+SELECT * FROM weekly_likes;
+```
+
+##### Update data held by materialized view 
+modifying any data in materialized view (SQL statement), will require us to refresh the materialized view to keep the data in-sync 
+
+```SQL 
+REFRESH MATERIALIZED VIEW  weekly_likes;
+```