第9课 汇总数据

• 9.1 聚集函数
  • AVG()平均值
  • COUNT()返回某列行数
  • MAX()返回某列最大值
  • MIN()返回某列最小值
  • SUM返回某列值之和
• 9.1.1 AVG( )函数
  • SELECT AVG(prod_price) AS avg_price
  • FROM products;
• 9.1.2 COUNT( )函数
  • SELECT COUNT(cust_email) AS num_cust
  • FROM Customers;
• 9.1.3 MAX( )函数
  • SELECT MAX(prod_price) AS max_price
  • FROM Products;
• 9.1.4 MIN( )函数
  • SELECT MIN(prod_price) AS min_price
  • FROM products;
• 9.1.5 SUM( )函数
  • /*SELECT SUM(quantity) AS items_ordered
  • FROM orderitems
  • WHERE order_num = 20005;*/
  • ---------------------------------------
  • SELECT SUM(item_price*quantity) AS total_price
  • FROM orderitems
  • WHERE order_num = 20005;
• 9.2 聚集不同值
  • SELECT AVG(DISTINCT prod_price) AS avg_price
  • FROM products
  • WHERE vend_id = 'DLL01';
• 9.3 组合聚集函数
  • SELECT COUNT(*) AS num_items,
  • MIN(prod_price) AS price_min,
  • MAX(prod_price) AS price_max,
  • AVG(prod_price) AS price_avg
  • FROM products;
• 9.4 小结

来自为知笔记(Wiz)