Great performance optimization tips

Author: ayushi-b

[Advanced] SQL Advanced Joins and Performance Tuning/Lesson 7 - [Advancerd] SQL Advanced Joins and Performance Tuning.sql

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+/*
+Write a query with FULL OUTER JOIN to see:
+- each account who has a sales rep and each sales rep that 
+has an account (all of the columns in these returned rows 
+will be full)
+- but also each account that does not have a sales rep and 
+each sales rep that does not have an account (some of the 
+columns in these returned rows will be empty)
+*/
+
+SELECT a.*,
+	s.id AS s_id,
+    s.name AS s_name,
+    s.region_id AS r_id
+FROM accounts a
+FULL OUTER JOIN sales_reps s 
+ON a.sales_rep_id = s.id;
+
+
+
+
+
+/*
+Write a query that left joins the accounts table and the 
+sales_reps tables on each sale rep's ID number and joins it 
+using the < comparison operator on accounts.primary_poc and 
+sales_reps.name, like so:
+	`accounts.primary_poc < sales_reps.name`
+The query results should be a table with three columns: the 
+account name (e.g. Johnson Controls), the primary contact 
+name (e.g. Cammy Sosnowski), and the sales representative's 
+name (e.g. Samuel Racine). 
+*/
+
+SELECT a.name,
+	a.primary_poc,
+    s.name AS sales_rep
+FROM accounts a
+LEFT JOIN sales_reps s
+ON a.sales_rep_id = s.id
+	AND a.primary_poc < s.name;
+
+
+
+
+
+/*
+Use SELF JOIN to find we events that occur one after another
+within 1 day for each account.
+*/
+
+SELECT w1.channel w1_channel,
+	w1.id w1_id,
+    w1.account_id w1_aid,
+	w1.occurred_at w1_time,
+	w2.channel w2_channel,
+    w2.id w2_id,
+    w2.account_id w2_aid,
+	w2.occurred_at w2_time
+FROM web_events w1
+LEFT JOIN web_events w2
+ON w1.account_id = w2.account_id
+	AND w1.occurred_at < w2.occurred_at
+    AND w1.occurred_at + INTERVAL '1 day' >= w2.occurred_at
+ORDER BY 1, 2;
+
+
+
+
+
+/*
+Write a query that uses UNION ALL on two instances (and selecting
+all columns) of the accounts table. 
+*/
+
+SELECT *
+FROM accounts
+
+UNION ALL
+
+SELECT *
+FROM accounts;
+
+
+
+
+
+/*
+Add a WHERE clause to each of the tables that you unioned in the 
+query above, filtering the first table where name equals Walmart 
+and filtering the second table where name equals Disney.
+*/
+
+SELECT *
+FROM accounts
+WHERE name = 'Walmart'
+
+UNION ALL
+
+SELECT *
+FROM accounts
+WHERE name = 'Disney';
+
+
+
+
+
+/*
+Perform the union in your first query (under the Appending Data via 
+UNION header) in a common table expression and name it double_accounts. 
+Then do a COUNT the number of times a name appears in the 
+double_accounts table. If you do this correctly, your query results 
+should have a count of 2 for each name.
+*/
+
+WITH double_accounts AS ( 
+  	SELECT *
+	FROM accounts
+	
+	UNION ALL
+
+	SELECT *	
+	FROM accounts
+)
+  
+SELECT name, COUNT(*)
+FROM double_accounts
+GROUP BY name;
+