more window functions

Author: mayankSystems

src/main/java/dev/mayankg/db_concepts/mysql/exc06/window_funcs.sql

@@ -1,23 +1,24 @@
--- VIEW ALL RECORDS
+-- VIEW ALL EMPLOYEE RECORDS
 SELECT * FROM employees;
 
--- MIN and MAX salary from the employees table
+-- CALCULATE OVERALL AVERAGE, MINIMUM, AND MAXIMUM SALARY ACROSS ALL EMPLOYEES
 SELECT
-	AVG(salary) AS avg_salary,
+    AVG(salary) AS avg_salary,
     MIN(salary) AS min_salary,
     MAX(salary) AS max_salary
 FROM employees;
 
--- SHOW EMPLOYEE DETAILS ALONG WITH OVERALL AVERAGE SALARY USING WINDOW FUNCTION
+-- SHOW EACH EMPLOYEE'S DETAILS WITH THE OVERALL AVERAGE SALARY USING A WINDOW FUNCTION
+-- Does not collapse rows — average appears on each row
 SELECT
     emp_no,
     department,
     salary,
     AVG(salary) OVER() AS avg_salary
 FROM employees;
 
--- AGGREGATE MIN AND MAX SALARIES (WITHOUT WINDOW FUNCTION) - RESULTS IN SINGLE ROW PER GROUP (IF GROUPED)
--- results in EXCEPTION if not grouped
+-- ⚠️ THIS QUERY IS COMMENTED OUT BECAUSE IT WOULD THROW AN ERROR
+-- IT TRIES TO USE AGGREGATE FUNCTIONS WITHOUT GROUPING OR A WINDOW FUNCTION
 -- SELECT
 --     emp_no,
 --     department,
@@ -26,16 +27,18 @@ FROM employees;
 --     MAX(salary) AS max_salary
 -- FROM employees;
 
--- AGGREGATE MIN AND MAX SALARIES (WITHOUT WINDOW FUNCTION) - RESULTS IN SINGLE ROW PER GROUP (IF GROUPED)
+-- SHOW AVERAGE, MINIMUM, AND MAXIMUM SALARIES PER DEPARTMENT
+-- GROUPS DATA BY DEPARTMENT AND RETURNS ONE ROW PER DEPARTMENT
 SELECT
     department,
-	AVG(salary) AS avg_salary,
+    AVG(salary) AS avg_salary,
     MIN(salary) AS min_salary,
     MAX(salary) AS max_salary
 FROM employees
 GROUP BY department;
 
--- SHOW EMPLOYEE DETAILS ALONG WITH MIN AND MAX SALARIES ACROSS ALL ROWS
+-- SHOW EACH EMPLOYEE'S DETAILS ALONG WITH OVERALL MIN AND MAX SALARIES USING WINDOW FUNCTION
+-- MIN() and MAX() are calculated once over the full table and shown with each row
 SELECT
     emp_no,
     department,
@@ -44,7 +47,8 @@ SELECT
     MAX(salary) OVER() AS max_salary
 FROM employees;
 
--- SHOW EMPLOYEE DETAILS ALONG WITH MIN AND MAX SALARIES PER DEPARTMENT
+-- SHOW EACH EMPLOYEE'S DETAILS WITH DEPARTMENT-WISE AVERAGE, MIN, AND MAX SALARIES
+-- Uses PARTITION BY department to group inside the window function
 SELECT
     emp_no,
     department,
@@ -54,11 +58,56 @@ SELECT
     MAX(salary) OVER(PARTITION BY department) AS dep_max_salary
 FROM employees;
 
--- SHOW EMPLOYEE DETAILS ALONG WITH MIN AND MAX SALARIES PER DEPARTMENT
+-- SHOW EACH EMPLOYEE'S DETAILS WITH DEPARTMENT-WISE AND OVERALL PAYROLL (TOTAL SALARY SUM)
+-- Uses SUM() as a window function for both scoped and global totals
 SELECT
     emp_no,
     department,
     salary,
     SUM(salary) OVER(PARTITION BY department) AS dept_payroll,
     SUM(salary) OVER() AS total_payroll
-FROM employees;
+FROM employees;
+
+-- SHOW EACH EMPLOYEE'S DETAILS ALONG WITH ROLLING SUM (CUMULATIVE) OF SALARIES PER DEPARTMENT
+-- Uses ORDER BY inside the window function to generate cumulative total by salary within each department
+SELECT
+    emp_no,
+    department,
+    salary,
+    SUM(salary) OVER(PARTITION BY department ORDER BY salary) AS rolling_dept_payroll,
+    SUM(salary) OVER(PARTITION BY department) AS dept_payroll
+FROM employees;
+
+-- SHOW EACH EMPLOYEE'S DETAILS WITH DEPARTMENT-WISE ROLLING MINIMUM SALARY IN DESCENDING ORDER
+-- Shows minimum salary seen so far as we move from highest to lowest in each department
+SELECT
+    emp_no,
+    department,
+    salary,
+    MIN(salary) OVER(PARTITION BY department ORDER BY salary DESC) AS rolling_min
+FROM employees;
+
+-- SHOW EMPLOYEE DETAILS ALONG WITH ROW NUMBER, RANK, AND DENSE_RANK FOR SALARIES
+-- Demonstrates use of window functions to rank employees within department and overall
+SELECT
+    emp_no,
+    department,
+    salary,
+
+    -- Assigns a unique row number per department based on descending salary
+    ROW_NUMBER() OVER(PARTITION BY department ORDER BY salary DESC) AS dept_row_number,
+
+    -- Ranks salaries within each department (duplicates get same rank, next rank is skipped)
+    RANK() OVER(PARTITION BY department ORDER BY salary DESC) AS dept_salary_rank,
+
+    -- Overall ranking across all departments (standard rank)
+    RANK() OVER(ORDER BY salary DESC) AS overall_rank,
+
+    -- Overall dense rank (no gaps in ranks for ties)
+    DENSE_RANK() OVER(ORDER BY salary DESC) AS overall_dense_rank,
+
+    -- Assigns a unique row number to each row based on overall descending salary
+    ROW_NUMBER() OVER(ORDER BY salary DESC) AS overall_num
+
+FROM employees
+ORDER BY overall_rank;