Question

我很难将多个表连接在一起；这样做时，我得到了意外的重复数据。

基本上，每个应用程序都可以具有一个或多个表单。每个申请人可以具有一个或多个表格。一个申请人可以拥有一个或多个地址；但是，为简单起见，我只想返回一个地址。

SELECT
    APPLICANT.FIRST_NAME,
    APPLICANT.LAST_NAME,
    ADDRESS.STREET,
    ADDRESS.CITY,
    ADDRESS.STATE,
    ADDRESS.POSTAL_CODE,
    FORM.SERIAL
FROM
    APPLICATION JOIN
    FORM ON
        FORM.APPLICATION_SERIAL = APPLICATION.SERIAL JOIN
    APPLICANT ON
        FORM.APPLICATION_SERIAL = APPLICANT.APPLICATION_SERIAL JOIN
    APPLICANT_ADDRESS ON
        APPLICANT.SERIAL = APPLICANT_ADDRESS.APPLICANT_SERIAL

运行此查询时，如果一个应用程序有两个申请人，每个申请人都有自己的地址，每个人都有一个表格，则每个申请人的结果重复两次。我尝试选择DISTINCT列以及按序列号，姓氏，名字等分组，但无济于事。

+------------+-----------+-----------------+--------------+-------+-------------+-------------+
| First_Name | Last_Name |     Street      |     City     | State | Postal_code | Form_Serial |
+------------+-----------+-----------------+--------------+-------+-------------+-------------+
| John       | Doe       | 123 Main Street | Magical City | ZZ    |       00000 |         001 |
| John       | Doe       | 123 Main Street | Magical City | ZZ    |       00000 |         001 |
| Jane       | Doe       | 456 Y Street    | Another City | ZZ    |       00000 |         002 |
| Jane       | Doe       | 456 Y Street    | Another City | ZZ    |       00000 |         002 |
+------------+-----------+-----------------+--------------+-------+-------------+-------------+

将这些表连接在一起时，防止重复行的最佳方法是什么？

Answer 1

在您的模式中，语句“每个申请人可以有一个或多个表单”不会遵循-Form实体与Application实体相关，但与Applicant不相关一。拥有重复的样本数据将是很好的。

WITH 
  APPLICATION (Serial) as (values
  (1)
) 
, FORM (Serial, Application_serial) as (values
  ('001', 1)
--, ('002', 1)
) 
, APPLICANT (Serial, Application_Serial, First_Name, Last_Name) as (values
  (1, 1, 'John', 'Doe')
, (2, 1, 'Jane', 'Doe')
)
, ADDRESS (Serial, Applicant_Serial, Street, City, State, Postal_code) as (values
  (1, 1, '123 Main Street', 'Magical City', 'ZZ', '00000')
, (2, 2, '456 Y Street', 'Another City', 'ZZ', '00000')
)
SELECT 
  APPLICANT.FIRST_NAME, APPLICANT.LAST_NAME
, ADDRESS.STREET, ADDRESS.CITY, ADDRESS.STATE, ADDRESS.POSTAL_CODE
, FORM.SERIAL AS FORM_SERIAL
FROM APPLICATION 
JOIN FORM ON FORM.APPLICATION_SERIAL = APPLICATION.SERIAL 
JOIN APPLICANT ON FORM.APPLICATION_SERIAL = APPLICANT.APPLICATION_SERIAL 
JOIN ADDRESS ON APPLICANT.SERIAL = ADDRESS.APPLICANT_SERIAL;

FIRST_NAME LAST_NAME STREET          CITY         STATE POSTAL_CODE FORM_SERIAL
---------- --------- --------------- ------------ ----- ----------- -----------
John       Doe       123 Main Street Magical City ZZ    00000       001
Jane       Doe       456 Y Street    Another City ZZ    00000       001

  2 record(s) selected.

我的示例不返回重复项。但是，如果您取消注释此Form的第二个Application（而不是Applicant），则会得到结果。也就是说，第二个Form记录是您在输出中得到重复项的原因。对于给定的FORM_SERIAL，如果有多个Forms，则应决定在Application列中返回什么。

DB2多表联接产生重复的行

1 个答案: