嵌套查询

是指在一个完整的查询语句之中，包含若干个不同功能的小查询；从而一起完成复杂查询的一种编写形式。包含的查询放在（）里 ， 包含的查询出现的位置：

位置	含义
SELECT之后	把查询结果作为表头使用
FROM之后	把查询结果作为表使用
WHERE之后	把查询结果作为查询条件
HAVING之后	把查询结果作为过滤使用

SELECT之后：

查看客户的总订单数

mysql> select cust_name,
    -> (select count(*) from orders
    -> where orders.cust_id = customers.cust_id )
    -> as orders_num from customers;
+----------------+------------+
| cust_name      | orders_num |
+----------------+------------+
| Coyote Inc.    |          2 |
| Mouse House    |          0 |
| Wascals        |          1 |
| Yosemite Place |          1 |
| E Fudd         |          1 |
+----------------+------------+
5 rows in set (0.01 sec)

WHERE之后：

查询下单了TNT2的客户id

mysql> select cust_id from orders
    -> where order_num in
    -> ( select order_num from orderitems
    -> where prod_id = 'TNT2' );
+---------+
| cust_id |
+---------+
|   10001 |
|   10004 |
+---------+
2 rows in set (0.01 sec)

实验：

1.使用子查询，返回购买价格为10或以上的商品的客户列表。您需，然后使用要使用Orderltems表查找匹配的订单号(order num)Orders表检索每个匹配订单的客户ID(cust id)。

mysql> select distinct cust_id from  orders
    -> where order_num in 
    -> ( select order_num from orderitems 
    -> where item_price >= 10 ) ;
+---------+
| cust_id |
+---------+
|   10001 |
|   10003 |
|   10004 |
+---------+
3 rows in set (0.01 sec)

2.您需要知道订购产品BR01的日期。编写一条SQL语句，使用子查询确定哪些订单(Orderltems中)购买了prod_id为BR01的商品，然后返回客户ID(cust_id)，和订单日期(order_date)。按订单日期排序结果。

mysql> select order_date from orders 
    -> where order_num in
    -> ( select order_num from orderitems
    -> where prod_id = 'ANV01');
+---------------------+
| order_date          |
+---------------------+
| 2023-09-01 00:00:00 |
+---------------------+
1 row in set (0.00 sec)

3.更新前面的挑战，为购买了prod id为AVN01的商品的任何客户返回客户电子邮件(Customers表中的custemail)。这里有一个提示:这涉及到SELECT语句，最里面的查询从Orderltems返回order num，中间的查询从Customers返回custid。

mysql> select cust_email from customers
    -> where cust_id in
    -> ( select cust_id from orders
    -> where order_num in
    -> ( select order_num from orderitems
    -> where prod_id = 'ANV01' ) );
+-----------------+
| cust_email      |
+-----------------+
| ylee@coyote.com |
+-----------------+
1 row in set (0.00 sec)

4.您需要一个包含每个客户订购的总额的客户ID列表。编写一条SOL语句，返回客户ID(0rders表中的cust id)和total ordered，并使用一个子查询返回每个客户的订单总数。按花费从大到小的顺序排列结果。这里有一个提示:你已经使用SUM()来计算订单总计。

mysql> SELECT cust_id,
    -> ( SELECT SUM(quantity * item_price )  FROM orderitems
    -> WHERE order_num IN
    -> ( SELECT order_num  FROM orders
    -> WHERE orders.cust_id = customers.cust_id ) ) 
    -> AS total_ordered
    -> FROM customers
    -> ORDER BY total_ordered DESC;
+---------+---------------+
| cust_id | total_ordered |
+---------+---------------+
|   10004 |       1000.00 |
|   10001 |        188.34 |
|   10005 |        125.00 |
|   10003 |         55.00 |
|   10002 |          NULL |
+---------+---------------+
5 rows in set (0.00 sec)

5.编写一条SQL语句，从Products表中检索所有产品名称(prodname)，以及一个名为quant_sold的计算列，该列包含此商品的销售总数(使用Orderltems表中的子查询和SUM(quantity)检索)。

mysql> SELECT prod_name,
    -> (SELECT Sum(quantity)
    -> FROM orderitems
    -> WHERE products.prod_id=orderitems.prod_id) 
    -> AS quant_sold
    -> FROM products;
+----------------+------------+
| prod_name      | quant_sold |
+----------------+------------+
| .5 ton anvil   |         10 |
| 1 ton anvil    |          3 |
| 2 ton anvil    |          1 |
| Detonator      |       NULL |
| Bird seed      |          2 |
| Carrots        |         50 |
| Fuses          |       NULL |
| JetPack 1000   |       NULL |
| JetPack 2000   |          1 |
| Oil can        |          1 |
| Safe           |       NULL |
| Sling          |          1 |
| TNT (1 stick)  |       NULL |
| TNT (5 sticks) |        105 |
+----------------+------------+
14 rows in set (0.00 sec)