window function and ordered data set

Author: ptyadana

Advanced SQL for Data Scientists/04.Window_Function_and_Ordered_Data.sql

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+/******** Windows Functions and Ordered Data ************/
+
+-- allows us to make sql statements about rows that are related to current row during processing.
+-- somewhat similar to Sub Query.
+
+
+--------------------- OVER (PARTITION BY) ---------------------------
+
+/* employee salary vs average salary of his/her department */
+SELECT 
+	department, 
+	last_name, 
+	salary,
+	AVG(salary) OVER (PARTITION BY department)
+FROM staff;
+
+
+/* employee salary vs max salary of his/her department */
+SELECT 
+	department, 
+	last_name, 
+	salary,
+	MAX(salary) OVER (PARTITION BY department)
+FROM staff;
+
+
+/* employee salary vs min salary of his/her Company Region */
+SELECT 
+	company_regions, 
+	last_name, 
+	salary,
+	MIN(salary) OVER (PARTITION BY company_regions)
+FROM vw_staff_div_reg;
+
+
+
+---------------------  FIRST_VALUE()  ---------------------------
+-- FIRST_VALUE returns first value of the partition conditions, in this case decending order of salary group by department
+
+SELECT
+	department,
+	last_name,
+	salary,
+	FIRST_VALUE(salary) OVER (PARTITION BY department ORDER BY salary DESC)
+FROM staff;
+
+
+/* this is same as above one, but above query is much cleaner and shorter */
+SELECT
+	department,
+	last_name,
+	salary,
+	MAX(salary) OVER (PARTITION BY department)
+FROM staff
+ORDER BY department ASC, salary DESC;
+
+---------------
+
+/* compare with the salary of person whose last name is in ascenidng in that department */
+SELECT
+	department,
+	last_name,
+	salary,
+	FIRST_VALUE(salary) OVER (PARTITION BY department ORDER BY last_name ASC)
+FROM staff;
+
+
+---------------------  RANK ()  ---------------------------
+
+-- give the rank by salary decending oder withint the specific department group.
+-- the ranking 1, 2, 3 will restart when it reaches to another unique group.
+-- works same as Row_Number Function
+SELECT
+	department,
+	last_name,
+	salary,
+	RANK() OVER (PARTITION BY department ORDER BY salary DESC)
+FROM staff;
+
+
+
+---------------------  ROW_NUBMER ()  ---------------------------
+-- same as above
+SELECT
+	department,
+	last_name,
+	salary,
+	ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC)
+FROM staff;	
+	
+
+--------------------- LAG() function ---------------------------
+-- to reference rows relative to the currently processed rows.
+-- LAG() allows us to compare condition with the previous row of current row.
+
+/* we want to know person's salary and next lower salary in that department */
+/* that is an additional column LAG. First row has no value because there is no previous value to compare.
+So it continues to next row and lag value of that second row will be the value of previous row, etc.
+It will restart again when we reache to another department.
+*/
+SELECT 
+	department,
+	last_name,
+	salary,
+	LAG(salary) OVER(PARTITION BY department ORDER BY salary DESC)
+FROM staff;
+
+
+
+--------------------- LEAD() function ---------------------------
+-- opposite of LAG()
+-- LEAD() allows us to compare condition with the next row of current row.
+-- now the last line of that department's LEAD value is empty because there is no next row value to compare.
+SELECT 
+	department,
+	last_name,
+	salary,
+	LEAD(salary) OVER(PARTITION BY department ORDER BY salary DESC)
+FROM staff;
+
+
+--------------------- NTILE(bins number) function ---------------------------
+-- allows to create bins/ bucket
+
+/* there are bins (1-10) assigned each employees based on the decending salary of specific department
+and bin number restart for another department agian */
+SELECT 
+	department,
+	last_name,
+	salary,
+	NTILE(10) OVER(PARTITION BY department ORDER BY salary DESC)
+FROM staff;
+