问题
Background:
Hi there, first time posting a question so please excuse any wrongdoings... I have been given an assignment at university using ASP.NET and SQL Server, what we basically have to do is create a webform where we can select a political party via a radio button list bound to a SQL data source. Based on this selection, a drop-down list is then populated with a list of MPs who are a part of the party selected the radio button list. You can then select an MP and view his or her personal details, their roles in parliament and ask/view questions via a number of data controls linked to various SQL data sources.
I'm making good progress and trying to use whatever resources I can find to get solutions but alas I've hit a wall with viewing the parliamentary roles after selecting the MP.
I have 3 tables which are involved here:
MP MP_ROLES ROLE
------------- --------------- ----------------
MP_NUM (PK) MP_NUM (PK, FK) ROLE_NUM (PK)
MP_FIRST ROLE_1 (FK) ROLE_DESCRIPTION
MP_LAST ROLE_2 (FK)
MP_EMAIL ROLE_3 (FK)
MP_CONTACTADD ROLE_4 (FK)
MP_PHOTO
PARTY_NUM(FK)
(where ROLE_1 through ROLE_4 reference ROLE.ROLE_NUM.)
Problem:
I have a DetailsView control which is bound to an SQL data source which queries MP_ROLES. What I'm basically trying to do is replace each value in ROLE_1, ROLE_2, etc. with the corresponding ROLE_DESCRIPTION value referenced by ROLE_NUM in the ROLE table (as MP_ROLE.ROLE_1 = ROLE.ROLE_NUM AND MP_ROLE.ROLE_2 = ROLE.ROLE_NUM AND MP_ROLE.ROLE_3 = ROLE.ROLE_NUM AND MP_ROLE.ROLE_4 = ROLE.ROLE_NUM). Now through my digging through textboxes I know in more simple applications you can do what I'm trying to do with the INNER JOIN function, however as I'm still learning I haven't quite got the syntax sorted to do what I want to do.
What I have so far:
SELECT MP_ROLES.MP_NUM, MP_ROLES.ROLE_1, MP_ROLES.ROLE_2, MP_ROLES.ROLE_3, MP_ROLES.ROLE_4
FROM MP_ROLES
INNER JOIN ROLE ON MP_ROLES.ROLE_1 = ROLE.ROLE_NUM
INNER JOIN ROLE AS ROLE_3 ON MP_ROLES.ROLE_2 = ROLE.ROLE_NUM
INNER JOIN ROLE AS ROLE_2 ON MP_ROLES.ROLE_3 = ROLE.ROLE_NUM
INNER JOIN ROLE AS ROLE_1 ON MP_ROLES.ROLE_4 = ROLE.ROLE_NUM
WHERE (MP_ROLES.MP_NUM = @MP_NUM)
I've been hung up on this for the last 8 trying to find a solution so if there is anyone who is able show me where I'm going wrong or perhaps kind enough to provide a solution it would be very much appreciated :)
回答1:
Your problem lies in the = ROLE.ROLE_NUM. You have to specify the alias here too. Also I rearranged some numbers since your numbers were all mixed up.
SELECT MP_ROLES.MP_NUM
, MP_ROLES.ROLE_1
, MP_ROLES.ROLE_2
, MP_ROLES.ROLE_3
, MP_ROLES.ROLE_4
FROM MP_ROLES
INNER JOIN ROLE AS ROLE_1 ON MP_ROLES.ROLE_1 = ROLE_1.ROLE_NUM
INNER JOIN ROLE AS ROLE_2 ON MP_ROLES.ROLE_2 = ROLE_2.ROLE_NUM
INNER JOIN ROLE AS ROLE_3 ON MP_ROLES.ROLE_3 = ROLE_3.ROLE_NUM
INNER JOIN ROLE AS ROLE_4 ON MP_ROLES.ROLE_4 = ROLE_4.ROLE_NUM
WHERE (MP_ROLES.MP_NUM = @MP_NUM)
回答2:
It looks as though what I was actually aiming for can be done using the LEFT OUTER JOIN function:
SELECT
MP_ROLES.MP_NUM,
MP_ROLES.ROLE_1,
MP_ROLES.ROLE_2,
MP_ROLES.ROLE_3,
MP_ROLES.ROLE_4,
ROLE1.ROLE_NUM, ROLE1.ROLE_DESCRIPTION AS POSITION1,
ROLE2.ROLE_NUM, ROLE2.ROLE_DESCRIPTION AS POSITION2,
ROLE3.ROLE_NUM, ROLE3.ROLE_DESCRIPTION AS POSITION3,
ROLE4.ROLE_NUM, ROLE4.ROLE_DESCRIPTION AS POSITION4
FROM MP_ROLES
LEFT OUTER JOIN ROLE AS ROLE1 ON MP_ROLES.ROLE_1 = ROLE1.ROLE_NUM
LEFT OUTER JOIN ROLE AS ROLE2 ON MP_ROLES.ROLE_2 = ROLE1.ROLE_NUM
LEFT OUTER JOIN ROLE AS ROLE3 ON MP_ROLES.ROLE_3 = ROLE1.ROLE_NUM
LEFT OUTER JOIN ROLE AS ROLE4 ON MP_ROLES.ROLE_4 = ROLE1.ROLE_NUM
WHERE (MP_ROLES.MP_NUM = @MP_NUM)
来源：https://stackoverflow.com/questions/19580570/multiple-column-sql-joins-in-a-table