CREATE TABLE `city` (
	`id` INT AUTO_INCREMENT PRIMARY KEY COMMENT '主鍵',
	`city_name` VARCHAR(100) COMMENT '城市名',
	`city_year` VARCHAR(4) COMMENT '年份，yyyy',
	`city_gdp` DOUBLE COMMENT '當前年份，城市一整年的GDP，單位億元',
	`city_population` DOUBLE COMMENT '當前年豐，城市的總人口，單位萬人'
);

INSERT INTO city VALUES(1,'上海',2018,32679,2418);
INSERT INTO city VALUES(2,'北京',2018,30320,2171);
INSERT INTO city VALUES(3,'深圳',2018,24691,1253);
INSERT INTO city VALUES(4,'廣州',2018,23000,1450);
INSERT INTO city VALUES(5,'重慶',2018,20363,3372);
INSERT INTO city VALUES(6,'上海',2019,38155,2424);
INSERT INTO city VALUES(7,'北京',2019,35371,2171);
INSERT INTO city VALUES(8,'深圳',2019,26927,1302);
INSERT INTO city VALUES(9,'廣州',2019,23628,1491);
INSERT INTO city VALUES(10,'重慶',2019,23605,3372);

SELECT COUNT(*) FROM city WHERE city_year = '2019';

SELECT MAX(city_gdp) FROM city WHERE city_year = '2018';

SELECT MIN(city_gdp) FROM city WHERE city_year = '2018';

SELECT SUM(city_gdp) FROM city WHERE city_year = '2019';

SELECT AVG(city_gdp) FROM city WHERE city_year = '2019';