Question

我是DAO世界的新手。我有3张桌子。

消费
比尔
收据

消费者表包含以下字段

consumer_id
CONSUMER_NAME
地址

比尔表包含

bill_number
bill_date
consumer_id
bill_amount

收据表包含

receipt_no
pay_date
paid_amount
consumer_id

比尔表与消费者表有 多对一 关系，收据 table还与消费者表有 多对一 关系。

目前我已经创建了三个类

消费
比尔
收据

为他们创建了DADA，例如ConsumerDAO，BillDAO，ReceiptDAO。它们包含基本的CRUD操作。

现在我想要一个涉及这三个表中数据的消费者列表。我正在使用JOIN SQL查询。它如下：

SELECT c.consumer_id,
       c.consumer_name,
       c.address,
       r.receipt_no,
       r.pay_date,
       r.paid_amount,
       b.bill_date,
       b.bill_number,
       b.bill_amount
FROM consumer c
LEFT OUTER JOIN
  (SELECT r.consumer_id,
          r.pay_date,
          r.receipt_number,
          r.paid_amount,
          ROW_NUMBER() OVER (PARTITION BY r.consumer_id
                             ORDER BY r.pay_date) AS rank
   FROM receipt r) r ON (c.consumer_id = r.consumer_id
                         AND r.rank = 1)
LEFT OUTER JOIN
  (SELECT b.consumer_id,
          b.bill_number,
          b.bill_date,
          b.bill_amount,
          ROW_NUMBER() OVER (PARTITION BY b.consumer_id
                             ORDER BY b.bill_date) AS rank
   FROM bill b) b ON (c.consumer_id = b.consumer_id
                      AND b.rank = 1)";

我想知道在哪个DAO我应该放置这个查询？我想在消费者类中添加 比尔字段 和 收据字段 并在 ConsumerDAO 中添加 getCurrentBillAndReceipt（） 方法。这是正确的做法吗？

我读了以下问题：

how to create a DAO for join tables

还有一些，但我无法理解。

Answer 1

如果您需要访问每位消费者的最新收单/收据，而不是尝试像这样进行查询，那么如果您的Consumer POJO成为以下内容会怎样：

public class Consumer {
    private Integer consumerId;
    private String consumerName;
    private String address;
    // join data into these variables
    private Set<Bill> bills = new Set<Bill>(0);
    private Set<Receipt> recipts = new Set<Receipt>(0);

    // add constructor and getter/setter methods here
}

然后，在ConsumerDAO中，您的find方法可能具有以下逻辑：

public class ConsumerDAO {
    /*
     * Find a consumer without any transaction data JOINed in
     */
    public Consumer find(Integer consumerId){
        // create a new Consumer object
        // run SQL query to populate consumerId, consumerName, and address fields of new Consumer object
        // return new Consumer object
    }

    /*
     * Find a consumer with all transaction data JOINed in
     */
    public Consumer findWithTransactionData(Integer consumerId){
        // create new consumer object
        // run a query to get the the basic fields of the Consumer POJO, that also uses
        // JOINs to get all of the consumer's Bills and Receipts; store all of the latter
        // in the Consumer object's Sets
        // return consumer object
    }

    /*** Note that the following two methods could even go in the Consumer class at this point ***/
    public Bill getMostRecentBill(Consumer consumer){
        // iterate through the consumer's bills, and return the most recent one
    }

    public Receipt getMostRecentReceipt(Consumer consumer){
       // iterate through the consumer's receipts, and return the most recent one
    }
}

这是你想要做的吗？我认为最有意义的是找到最新的＆＃34; Consumer类本身的方法，因为您不必将任何内容作为参数传递给方法;如果您愿意，可以在getMostRecentBill()上的Consumer对象上调用findWithTransactionData(Integer)。然而，这通常被认为是一种糟糕的做法，因为POJO应该是愚蠢的＃34;尽可能。因此，您希望如何处理这一点取决于您。

DAO涉及JOIN查询

1 个答案: