MySQL必知必会读书笔记-4（数据分组和子查询）

Posted 2020-08-20 vplus

1--创建分组

列出每个供应商的产品数量

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

 

2--过滤分组

显示有两个以上订单的顾客

SELECT cust_id, COUNT(*) AS orders
FROM orders
GROUP BY cust_id
HAVING COUNT(*) >= 2

 

3--HAVING和WHERE

 

显示出有两个以上的价值在10以上的物品的供应商

1.筛选出所有价格在10以上的商品  2.根据供应商id分组  3.找出数量大于等于2的分组

SELECT vend_id, COUNT(*) AS num_prods
FROM products
WHERE prod_price >= 10
GROUP BY vend_id
HAVING COUNT(*)>=2

 

 4--分组和排序

检索出总计价格>=50的订单和订单总计价格，按订单总计价格排序输出

1.根据订单号分组  2.计算每个分组的总价格，即每个订单的总价  3.过滤掉总价低于50的订单  4.根据订单总价排序

SELECT order_num, SUM(quantity*item_price) AS ordertotal
FROM orderitems
GROUP BY order_num
HAVING SUM(quantity*item_price)>=50
ORDER BY ordertotal

 

5--select子句顺序

   

 

6--子查询

执行子查询的时候，总是由内到外执行

查询订购了TNT2的所有顾客的信息

（1）检索出包含TNT2的所有订单编号

SELECT order_num
FROM orderitems
WHERE prod_id = "TNT2"

（2）根据订单编号找出用户ID

SELECT cust_id
FROM orders
WHERE order_num IN (20005, 20007)

（3）根据用户ID查询该用户信息

SELECT cust_id, cust_name
FROM customers
WHERE cust_id IN (10001, 10004)

使用子查询

SELECT cust_id, cust_name
FROM customers
WHERE cust_id IN (SELECT cust_id
                  FROM orders
                  WHERE order_num IN (
                                       SELECT order_num
                                       FROM orderitems
                                       WHERE prod_id = "TNT2"
                                     )
                 )

 

7--作为计算字段使用子查询

显示每个顾客的订单数

 （1）显示一个顾客的订单数

SELECT COUNT(*) AS ordernum
FROM orders
WHERE cust_id = 10001

（2）对每一个客户进行COUNT（*）计算来得到每一个用户的订单数，应将COUNT（*）作为一个子查询

SELECT cust_name, (
                   SELECT COUNT(*)
                   FROM orders
                   WHERE orders.cust_id = customers.cust_id
                  ) AS orders
FROM customers

这是一个相关子查询（涉及外部查询的子查询，外部查询每查询一条记录就要执行一次子查询）