合併使用1三個查詢：*關係到一個查詢

Question

可以說我有四個表，我想閱讀：

客戶

customer_id, customer_name 
1    Joe Bolggs 

customer_orders

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

customer_addresses

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

customer_tel_no

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

上面所示的客戶信息（與ID = 1的顧客）要被存儲在Java對象如下所示

public class Customer{ 

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

唯一我可以想到的方式來獲得上述信息是通過使用三個查詢。原因是客戶表和其他每個表之間存在1：*關係。要得到的數據我這樣做是這樣的：

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 
} 

上述問題是，我正在對數據庫進行三次調用。有沒有辦法可以將上述內容合併到一個查詢中？

我不能看到一個連接如何工作，因爲例如，customer和customer_orders之間的連接將爲customer_orders中的每一行返回一個客戶行。無論如何，我可以將上述三個查詢合併爲一個？

Answer 1

我認爲，像這樣的工作：

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 = ? 

然後，在你的代碼，執行查詢後：

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

當然，這並沒有考慮到事實上，你會一路上有空值，所以我建議檢查空值，以確保你沒有添加很多垃圾到你的數組列表中。不過，這可能比3個單獨的查詢花費少很多。

Answer 2

沒有什麼能阻止你迭代和處理連接結果到你的客戶對象。如果您的應用程序足夠複雜，那麼您可以查看ORM框架，這些框架可以爲您解決這個問題。如果您正在使用JavaEE，請查看JPA。

Answer 3

使用此查詢並減少呼叫次數。並且，在數據的while循環過程中。

選擇customer.customer_id，customer.customer_name，order_no，地址，數

從顧客，customer_orders，customer_addresses，customer_tel_no

其中customer.customer_id = customer_orders。CUSTOMER_ID 和 customer.customer_id = customer_addresses.customer_id AND customer.customer_id = customer_tel_no.customer_id