completed till GROUPING SETS

Complete SQL & Databases - ZTM/06_Advanced_SQL.sql

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+/******** 1) GROUP BY  **********/
+/*
+	When we group by, we apply the function PER GROUP,	NOT on the ENTIRE DATA SET.
+	Group by use Split, Apply, Combine strategry.
+*/
+
+/* How many employees worked in each department ? */
+SELECT d.dept_name AS "Department Name" ,COUNT(e.emp_no) AS "Number Of Employee"
+FROM employees e
+JOIN dept_emp de ON e.emp_no = de.emp_no
+JOIN departments d ON d.dept_no = de.dept_no
+GROUP BY d.dept_no
+ORDER BY 1;
+
+
+/************ 2) HAVING Keyword *************/
+/*
+	"Having" applies filters to a group as a whole
+	
+	**** Order of Operations ****
+		FROM
+		WHERE
+		GROUP BY
+		HAVING
+		SELECT
+		ORDER
+*/
+
+/* How many employees worked in each department, but with employees more than 25000 ? */
+SELECT d.dept_name AS "Department Name" ,COUNT(e.emp_no) AS "Number Of Employee"
+FROM employees e
+JOIN dept_emp de ON e.emp_no = de.emp_no
+JOIN departments d ON d.dept_no = de.dept_no
+GROUP BY d.dept_name
+HAVING COUNT(e.emp_no) > 25000
+ORDER BY 1;
+
+/* How many Female employees worked in each department, but with employees more than 25000 ? */
+SELECT d.dept_name AS "Department Name" ,COUNT(e.emp_no) AS "Number Of Employee"
+FROM employees e
+JOIN dept_emp de ON e.emp_no = de.emp_no
+JOIN departments d ON d.dept_no = de.dept_no
+WHERE e.gender='F'
+GROUP BY d.dept_name
+HAVING COUNT(e.emp_no) > 25000
+ORDER BY 1;
+
+
+/********** 3) Ordering Group Data **********/
+SELECT d.dept_name AS "Department Name" ,COUNT(e.emp_no) AS "Number Of Employee"
+FROM employees e
+JOIN dept_emp de ON e.emp_no = de.emp_no
+JOIN departments d ON d.dept_no = de.dept_no
+GROUP BY d.dept_name
+HAVING COUNT(e.emp_no) > 25000
+ORDER BY 2 DESC;
+
+
+/********* 4) Group by Mental Model ***********/
+/* What are the 8 employees who got the most salary bumps? */
+-- SELECT e.emp_no, CONCAT(e.first_name, e.last_name) AS "Name", s.salary, s.from_date, s.to_date
+SELECT emp_no, MAX(from_date)
+FROM salaries
+GROUP BY emp_no;
+
+/*********** 5) GROUPING SETS**********/
+
+/******* UNION / UNION ALL *********/
+/*
+	SELECT col1, SUM(col2)
+	FROM table
+	GROUP BY col1
+	
+	UNION / UNION ALL
+	
+	SELECT SUM(col2)
+	FROM table
+	
+	
+ 	UNION ALL doesn't remove DUPLICATE Records.
+*/
+SELECT NULL AS "prod_id", sum(ol.quantity)
+FROM orderlines AS ol
+UNION
+SELECT prod_id AS "prod_id", sum(ol.quantity)
+FROM orderlines AS ol
+GROUP BY prod_id
+ORDER BY prod_id DESC;
+
+/*********** GROUPING SETS ***********/
+/* 
+	A Subclause of GROUP BY that allows you to define multiple grouping
+	It is very useful when we want to combine multiple grouping 
+*/
+
+-- same result as using above UNION code
+SELECT NULL AS "prod_id", sum(ol.quantity)
+FROM orderlines AS ol
+GROUP BY
+	GROUPING SETS(
+		(),
+		(prod_id)
+	)
+ORDER BY prod_id DESC;
+
+/* we can add in multiple groups as we need */
+SELECT NULL AS "prod_id", orderlineid, sum(ol.quantity)
+FROM orderlines AS ol
+GROUP BY
+	GROUPING SETS(
+		(),
+		(prod_id),
+		(orderlineid)
+	)
+ORDER BY prod_id DESC, orderlineid DESC;