MySQL 8.0

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CREATE TABLE city (id INT, countrycode INT, population INT);
CREATE TABLE country (code INT, population INT);
CREATE TRIGGER diff_pop_ai
AFTER INSERT 
ON city
FOR EACH ROW
UPDATE country 
SET population = population + NEW.population 
WHERE country.code = NEW.countrycode;
CREATE TRIGGER diff_pop_au
AFTER UPDATE 
ON city
FOR EACH ROW
UPDATE country 
SET population = population - OLD.population + NEW.population 
WHERE country.code = NEW.countrycode;
CREATE TRIGGER diff_pop_ad
AFTER DELETE 
ON city
FOR EACH ROW
UPDATE country 
SET population = population - OLD.population 
WHERE country.code = OLD.countrycode;
INSERT INTO country VALUES (1,0), (2,0);

Records: 2  Duplicates: 0  Warnings: 0


INSERT INTO city VALUES (1,1,100);
INSERT INTO city VALUES (2,2,200);
INSERT INTO city VALUES (3,1,300);
SELECT * FROM city;
SELECT * FROM country;


id
countrycode
population


1
1
100

2
2
200

3
1
300




code
population


1
400

2
200


UPDATE city SET population = 400 WHERE id = 2;
UPDATE city SET population = 500 WHERE id = 3;
SELECT * FROM city;
SELECT * FROM country;

Rows matched: 1  Changed: 1  Warnings: 0



Rows matched: 1  Changed: 1  Warnings: 0




id
countrycode
population


1
1
100

2
2
400

3
1
500




code
population


1
600

2
400


DELETE FROM city WHERE id = 3;
SELECT * FROM city;
SELECT * FROM country;


id
countrycode
population


1
1
100

2
2
400




code
population


1
100

2
400