DQL-聚合函数GroupBy

# 1.聚合函数的使用
# 求所有手机的价格的总和
SELECT SUM(price) totalPrice FROM `products`;
# 求一下华为手机的价格的总和
SELECT SUM(price) FROM `products` WHERE brand = '华为';
# 求华为手机的平均价格
SELECT AVG(price) FROM `products` WHERE brand = '华为';
# 最高手机的价格和最低手机的价格
SELECT MAX(price) FROM `products`;
SELECT MIN(price) FROM `products`;

# 求华为手机的个数
SELECT COUNT(*) FROM `products` WHERE brand = '华为';
SELECT COUNT(*) FROM `products` WHERE brand = '苹果';
SELECT COUNT(url) FROM `products` WHERE brand = '苹果';

SELECT COUNT(price) FROM `products`;
SELECT COUNT(DISTINCT price) FROM `products`;

# 2.GROUP BY的使用
SELECT brand, AVG(price), COUNT(*), AVG(score) FROM `products` GROUP BY brand;

# 3.HAVING的使用
SELECT brand, AVG(price) avgPrice, COUNT(*), AVG(score) FROM `products` GROUP BY brand HAVING avgPrice > 2000;

# 4.需求：求评分score > 7.5的手机的，平均价格是多少？
# 升级：平均分大于7.5的手机，按照品牌进行分类，求出平均价格。
SELECT brand, AVG(price) FROM `products` WHERE score > 7.5 GROUP BY brand;