MySQL实践-子查询

子查询

MySQL 4.1以上版本引入了子查询的概念，即允许在一条查询语句中嵌套其他的查询语句，子查询语句的返回结果可以作为上级查询的查询条件。
本节原始数据：

mysql> SELECT * FROM orderitems;
+-----------+------------+---------+----------+------------+
| order_num | order_item | prod_id | quantity | item_price |
+-----------+------------+---------+----------+------------+
|     20005 |          1 | ANV01   |       10 |       5.99 |
|     20005 |          2 | ANV02   |        3 |       9.99 |
|     20005 |          3 | TNT2    |        5 |      10.00 |
|     20005 |          4 | FB      |        1 |      10.00 |
|     20006 |          1 | JP2000  |        1 |      55.00 |
|     20007 |          1 | TNT2    |      100 |      10.00 |
|     20008 |          1 | FC      |       50 |       2.50 |
|     20009 |          1 | FB      |        1 |      10.00 |
|     20009 |          2 | OL1     |        1 |       8.99 |
|     20009 |          3 | SLING   |        1 |       4.49 |
|     20009 |          4 | ANV03   |        1 |      14.99 |
+-----------+------------+---------+----------+------------+
11 rows in set (0.00 sec)

mysql> SELECT * FROM orders;
+-----------+---------------------+---------+
| order_num | order_date          | cust_id |
+-----------+---------------------+---------+
|     20005 | 2005-09-01 00:00:00 |   10001 |
|     20006 | 2005-09-12 00:00:00 |   10003 |
|     20007 | 2005-09-30 00:00:00 |   10004 |
|     20008 | 2005-10-03 00:00:00 |   10005 |
|     20009 | 2005-10-08 00:00:00 |   10001 |
+-----------+---------------------+---------+
5 rows in set (0.00 sec)

mysql> SELECT * FROM customers;
+---------+----------------+---------------------+-----------+------------+----------+--------------+--------------+---------------------+
| cust_id | cust_name      | cust_address        | cust_city | cust_state | cust_zip | cust_country | cust_contact | cust_email          |
+---------+----------------+---------------------+-----------+------------+----------+--------------+--------------+---------------------+
|   10001 | Coyote Inc.    | 200 Maple Lane      | Detroit   | MI         | 44444    | USA          | Y Lee        | [email protected]     |
|   10002 | Mouse House    | 333 Fromage Lane    | Columbus  | OH         | 43333    | USA          | Jerry Mouse  | NULL                |
|   10003 | Wascals        | 1 Sunny Place       | Muncie    | IN         | 42222    | USA          | Jim Jones    | [email protected] |
|   10004 | Yosemite Place | 829 Riverside Drive | Phoenix   | AZ         | 88888    | USA          | Y Sam        | [email protected]    |
|   10005 | E Fudd         | 4545 53rd Street    | Chicago   | IL         | 54545    | USA          | E Fudd       | NULL                |
+---------+----------------+---------------------+-----------+------------+----------+--------------+--------------+---------------------+
5 rows in set (0.00 sec)

利用子查询进行过滤

如果要查询出订购了物品TNT2的所有客户，在不使用子查询的条件下，查询步骤如下：
1. 从orderitems表中查询出包含物品TNT2的所有订单的编号

mysql> SELECT order_num FROM orderitems WHERE prod_id = 'TNT2';
+-----------+
| order_num |
+-----------+
|     20005 |
|     20007 |
+-----------+
2 rows in set (0.00 sec)

1. 从orders表中查询具有上一步骤列出的订单编号的所有客户id

mysql> SELECT cust_id FROM orders WHERE order_num IN (20005,20007);
+---------+
| cust_id |
+---------+
|   10001 |
|   10004 |
+---------+
2 rows in set (0.00 sec)

1. 从customers表中根据上一步骤的出的客户id查询客户信息

mysql> SELECT cust_name, cust_contact FROM customers WHERE cust_id IN (10001,10004);
+----------------+--------------+
| cust_name      | cust_contact |
+----------------+--------------+
| Coyote Inc.    | Y Lee        |
| Yosemite Place | Y Sam        |
+----------------+--------------+
2 rows in set (0.00 sec)

使用子查询进行过滤的话，只需一条语句：

mysql> SELECT cust_name, cust_contact
    -> FROM customers
    -> WHERE cust_id IN (SELECT cust_id 
    ->                   FROM orders
    ->                   WHERE order_num IN (SELECT order_num
    ->                                       FROM orderitems
    ->                                       WHERE prod_id = 'TNT2'));
+----------------+--------------+
| cust_name      | cust_contact |
+----------------+--------------+
| Coyote Inc.    | Y Lee        |
| Yosemite Place | Y Sam        |
+----------------+--------------+
2 rows in set (0.00 sec)

作为计算字段使用子查询

假设需要显示customers表中每个客户的订单数，步骤如下 ：
1. 从customers表中检索客户列表
2. 针对上一步骤中查询出的每个客户，统计其在orders表中的订单数

对表中的行进行计数可以使用COIUNT（*）,为了针对每个客户执行COUNT(*)计算，应该将COUNT(*)作为一个子查询：

mysql> SELECT cust_name,cust_state,(SELECT COUNT(*) FROM orders WHERE orders.cust_id = customers.cust_id) AS orders 
    -> FROM customers ORDER BY cust_name;
+----------------+------------+--------+
| cust_name      | cust_state | orders |
+----------------+------------+--------+
| Coyote Inc.    | MI         |      2 |
| E Fudd         | IL         |      1 |
| Mouse House    | OH         |      0 |
| Wascals        | IN         |      1 |
| Yosemite Place | AZ         |      1 |
+----------------+------------+--------+
5 rows in set (0.00 sec)

上面的子查询针对每个检索出的客户计算一次，在这里子查询必须使用完全限定列名（表名.列名）的形式。