mysql-6 数据检索（4）

汇总数据

函数	说明
AVG()	返回某列的平均数
COUNT()	返回某列的行数
MAX()	返回某列的最大值
MIN()	返回某列的最小值
SUM()	返回某列值的和

1、AVG函数

SELECT AVG(prod_price) AS avg_price FROM products;

SELECT AVG(prod_price) AS avg_price FROM products WHERE vend_id = ;

2、count()函数

SELECT COUNT(*) AS num_cust FROM customers;

SELECT COUNT(cust_email) AS num_cust FROM customers;

这条select语句使用count(cust_email)对cust_email列中有值的进行计数，在此例子中，cust_email的计算式3次，表示5个客户中只有三个客户有电子邮件

3、max()函数

SELECT MAX(prod_price) AS max_price FROM products;

4、min()函数

SELECT MIN(prod_price) AS min_price FROM products;

5、sum()函数

SELECT SUM(quantity) AS items_ordered FROM orderitems WHERE order_num = ;

SELECT SUM(item_price*quantity) AS total_price FROM orderitems WHERE order_num = ;

SELECT COUNT(*) AS num_items,MIN(prod_price) AS price_min, MAX(prod_price) AS price_max,AVG(prod_price) AS avg_price FROM products;

分组数据    group by

SELECT vend_id ,COUNT(*) AS num_prods FROM products GROUP BY vend_id;

SELECT cust_id ,COUNT(*) AS orders FROM orders GROUP BY cust_id HAVING COUNT(*)>=;

SELECT prod_price,vend_id,COUNT(*) AS num_prods FROM products WHERE prod_price >= GROUP BY vend_id HAVING COUNT(*)>=;

SELECT vend_id, COUNT(*) AS num_prods FROM products GROUP BY vend_id HAVING COUNT(*)>=;

order by 1、排序产生的输出 2、任意列都可以使用 3、不一定需要

group by 1、分组行，但输出可能不是分组的顺序 2、只可能使用选择列或表达式列，而且必须使用每个选择列表达式 3、如果与聚集函数一起使用列，则必须使用

where过滤行，having过滤组

SELECT order_num ,SUM(quantity*item_price) AS ordertotal FROM orderitems GROUP BY order_num HAVING SUM(quantity*item_price)>=;

SELECT order_num ,SUM(quantity*item_price) AS ordertotal FROM orderitems GROUP BY order_num HAVING SUM(quantity*item_price)>= ORDER BY ordertotal;

使用子查询

SELECT order_num FROM orderitems WHERE prod_id = 'TNT2';
SELECT cust_id FROM orders WHERE order_num IN (,);
SELECT cust_id FROM orders WHERE order_num IN (SELECT order_num FROM orderitems WHERE prod_id = 'TNT2');

SELECT AVG(prod_price) *AS avg_price FROM products;
SELECT AVG(prod_price) AS avg_price FROM products WHERE vend_id = ;
SELECT COUNT(*) AS num_cust FROM customers;
SELECT COUNT(cust_email) AS num_cust FROM customers;
SELECT MAX(prod_price) AS max_price FROM products;
SELECT MIN(prod_price) AS min_price FROM products;
SELECT SUM(quantity) AS items_ordered FROM orderitems WHERE order_num = ;
SELECT SUM(item_price*quantity) AS total_price FROM orderitems WHERE order_num = ;
SELECT COUNT(*) AS num_items,MIN(prod_price) AS price_min, MAX(prod_price) AS price_max,AVG(prod_price) AS avg_price FROM products;
SELECT vend_id ,COUNT(*) AS num_prods FROM products GROUP BY vend_id;
SELECT cust_id FROM orders ;
SELECT cust_id ,COUNT(*) AS orders FROM orders GROUP BY cust_id HAVING COUNT(*)>=;
SELECT prod_price,vend_id,COUNT(*) AS num_prods FROM products WHERE prod_price >= GROUP BY vend_id HAVING COUNT(*)>=;
SELECT vend_id, COUNT(*) AS num_prods FROM products GROUP BY vend_id HAVING COUNT(*)>=;
SELECT order_num ,SUM(quantity*item_price) AS ordertotal FROM orderitems GROUP BY order_num HAVING SUM(quantity*item_price)>=;
SELECT order_num ,SUM(quantity*item_price) AS ordertotal FROM orderitems GROUP BY order_num HAVING SUM(quantity*item_price)>= ORDER BY ordertotal;
SELECT order_num FROM orderitems WHERE prod_id = 'TNT2';
SELECT cust_id FROM orders WHERE order_num IN (,);
SELECT cust_id FROM orders WHERE order_num IN (SELECT order_num FROM orderitems WHERE prod_id = 'TNT2');