0

假设我有四个要读取的表：

顾客

customer_id,    customer_name
1               Joe Bolggs

客户订单

customer_id, order_no
----------------------------
1       1
1       2
1       3

客户地址

customer_id address
----------------------------
1       11 waterfall road
1       23 The broadway

customer_tel_no

customer_id number
----------------------------
1       523423423432
1       234342342343

上面显示的客户信息（对于 id=1 的客户）将存储在 Java 对象中，如下所示

public class Customer{

    String customer_id;
    String customerName;
    ArrayList<String> customerOrders;
    ArrayList<String> customerAddress;
    ArrayList<String> customerTelephoneNumbers;
}

我能想到的获得上述信息的唯一方法是使用三个查询。原因是客户表和其他每个表之间存在 1:* 关系。为了获取数据，我正在做这样的事情：

Customer customer = new Customer()

String customerSQL = "Select * from customer where customer_id = ?";
statement = getConnection().prepareStatement(contactsQuery);
statement.setString(1,1);
resultSet = statement.executeQuery(); 

while (resultSet.next()){
    customer.customer_id = resultSet.get(1);    //No getters/setters in this example
    customer.customerName = resultSet.get(2);
}

String customerOrdersSQL = "Select * from customer_orders where customer_id = ?";
statement = getConnection().prepareStatement(customerOrdersSQL); 
statement.setString(1,1);
resultSet = statement.executeQuery(); 

customer.customerOrders = new ArrayList();
while (resultSet.next()){
    customer.customerOrders.add(resultSet.getString(2);  // all the order numbers
}


String customerAddressesSQL = "Select * from customer_addresses where customer_id = ?";
statement = getConnection().prepareStatement(customerAddressesSQL ); 
statement.setString(1,1);
resultSet = statement.executeQuery(); 

customer.customerAddresses = new ArrayList();
while (resultSet.next()){
    customer.customerAddresses.add(resultSet.getString(2);  // all the addresses
}


String customerTelSQL = "Select * from customer_tel_no where customer_id = ?";
statement = getConnection().prepareStatement(customerTelSQL); 
statement.setString(1,1);
resultSet = statement.executeQuery(); 

customer.customerTelephoneNumbers = new ArrayList();
while (resultSet.next()){
    customer.customerTelephoneNumbers.add(resultSet.getString(2);  // all the order numbers
}

上述问题是我正在对数据库进行三个调用。请问有没有办法可以将以上内容合并到一个查询中？

我看不到联接是如何工作的，因为例如，客户和客户订单之间的联接将为客户订单中的每一行返回一个客户行。无论如何我可以将上述三个查询合并为一个吗？

4

3 回答 3

1

我认为这样的事情会起作用：

SELECT c.customer_id, c.customer_name, o.order_no, a.address, t.number
FROM customer c LEFT JOIN customer_orders o ON c.customer_id = o.customer_id
LEFT JOIN customer_addresses a ON c.customer_id = a.customer_id
LEFT JOIN customer_tel_no t ON c.customer_id = t.customer_id
WHERE c.customer_id = ?

然后，在您的代码中，执行查询后：

while (resultSet.next())
{
    customer.customerOrders.add(resultSet.getString(3));
    customer.customerAddresses.add(resultSet.getString(4));
    customer.customerTelephoneNumbers.add(resultSet.getString(5));
}

当然，这并没有考虑到您在此过程中会有空值这一事实，因此我建议您检查空值以确保您不会在数组列表中添加大量垃圾。不过，这可能比 3 个单独的查询便宜得多。

于 2012-11-30T20:19:46.433 回答
1

没有什么可以阻止您将连接的结果迭代和处理到您的客户对象中。如果您的应用程序足够复杂，您可以研究 ORM 框架，它会在幕后为您做到这一点。如果您正在使用 JavaEE，请查看 JPA。

于 2012-11-30T20:23:56.410 回答
0

使用此查询并减少调用次数。并且，在数据的while循环过程中。

选择 customer.customer_id,customer.customer_name,order_no,address,number

来自客户、customer_orders、customer_addresses、customer_tel_no

其中 customer.customer_id = customer_orders.customer_id AND customer.customer_id = customer_addresses.customer_id AND customer.customer_id = customer_tel_no.customer_id

于 2012-12-28T14:28:44.437 回答