https://www.youtube.com/watch?v=60MJSk56MZs&list=PLavw5C92dz9Hxz0YhttDniNgKejQlPoAn&index=16

STR_TO_DATE()

DROP TABLE IF EXISTS covid_cases;

CREATE TABLE covid_cases (
    cases_reported INT,
    dates DATE
);

INSERT INTO covid_cases VALUES (20124, STR_TO_DATE('10/01/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (40133, STR_TO_DATE('15/01/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (65005, STR_TO_DATE('20/01/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (30005, STR_TO_DATE('08/02/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (35015, STR_TO_DATE('19/02/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (15015, STR_TO_DATE('03/03/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (35035, STR_TO_DATE('10/03/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (49099, STR_TO_DATE('14/03/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (84045, STR_TO_DATE('20/03/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (100106, STR_TO_DATE('31/03/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (17015, STR_TO_DATE('04/04/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (36035, STR_TO_DATE('11/04/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (50099, STR_TO_DATE('13/04/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (87045, STR_TO_DATE('22/04/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (101101, STR_TO_DATE('30/04/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (40015, STR_TO_DATE('01/05/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (54035, STR_TO_DATE('09/05/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (71099, STR_TO_DATE('14/05/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (82045, STR_TO_DATE('21/05/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (90103, STR_TO_DATE('25/05/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (99103, STR_TO_DATE('31/05/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (11015, STR_TO_DATE('03/06/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (28035, STR_TO_DATE('10/06/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (38099, STR_TO_DATE('14/06/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (45045, STR_TO_DATE('20/06/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (36033, STR_TO_DATE('09/07/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (40011, STR_TO_DATE('23/07/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (25001, STR_TO_DATE('12/08/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (29990, STR_TO_DATE('26/08/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (20112, STR_TO_DATE('04/09/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (43991, STR_TO_DATE('18/09/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (51002, STR_TO_DATE('29/09/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (26587, STR_TO_DATE('25/10/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (11000, STR_TO_DATE('07/11/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (35002, STR_TO_DATE('16/11/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (56010, STR_TO_DATE('28/11/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (15099, STR_TO_DATE('02/12/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (38042, STR_TO_DATE('11/12/2020', '%d/%m/%Y'));
INSERT INTO covid_cases VALUES (73030, STR_TO_DATE('26/12/2020', '%d/%m/%Y'));

SELECT * FROM covid_cases;

WITH
LAG()
CASE WHEN

-- Could've been done in 2 ctes, Using the lag in the cte final with the expression itself.
WITH Original_nums(Month_Num,Monthly_Cases) AS (
SELECT MONTH(dates) AS Month_Num,SUM(cases_reported) AS Monthly_Cases FROM covid_cases
GROUP BY MONTH(dates)
),
cte2 AS (
SELECT *,
SUM(Monthly_Cases) OVER (ORDER BY Month_Num) AS Total_Cases 
FROM Original_Nums
),
cte3 AS (
SELECT Month_Num AS Month, 
Monthly_cases,
LAG(Total_cases,1) OVER (ORDER BY Month_Num) AS la_g
FROM cte2
)
SELECT Month,
CASE WHEN Month>1 THEN ((Monthly_cases / la_g)*100) 
ELSE '-' END AS Percentage_increase
FROM cte3