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)
)
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