sql server 2008 case statement in where clause not

问题:

I have 3 table, Documents, SR and events

In documents I am saving all documents related to SR and Events.

And I want to show all documents in one page.

So I am using this select query

Select * 
from documents d, SR s, Events e 
where d.relationid = ( case d.documenttype when 'SR' the s.SRId else 'e.eventid end) 

but it's not working.

My document table structure is like this :

documentid int, 
documenttype nvarchar(50),
relationid int,
document image,
docdate date

Can anyone please point me to my mistake ?

I want to select all documents with related info. means if its SR document than SR details should display otherwise Events. There is only 2 types of documents right now.

What should be my select query for this ?

回答1:

You can join then using LEFT JOIN,

SELECT  d.*,
        COALESCE(s.Col1, e.Col1) AS Col1,
        COALESCE(s.Col2, e.Col2) AS Col2,
        COALESCE(s.Col3, e.Col3) AS Col3,
        COALESCE(s.Col4, e.Col4) AS Col4
FROM    documents d
        LEFT JOIN SR s
            ON d.relationID = d.SRID
        LEFT JOIN Events e
            ON d.relationID = e.eventID

where Col1,...., Col4 are the columns of each table youw ant to be displayed based on documenttype.

To further gain more knowledge about joins, kindly visit the link below:

• Visual Representation of SQL Joins

The more safe version of the query above assuming that the same ID can contain on SR and Events table would be by using CASE()

SELECT  d.*,
        CASE WHEN d.documenttype = 'SR' THEN s.Col1 ELSE e.Col1 END) AS Col1,
        CASE WHEN d.documenttype = 'SR' THEN s.Col2 ELSE e.Col2 END) AS Col2,
        CASE WHEN d.documenttype = 'SR' THEN s.Col3 ELSE e.Col3 END) AS Col3,
        CASE WHEN d.documenttype = 'SR' THEN s.Col4 ELSE e.Col4 END) AS Col4
FROM    documents d
        LEFT JOIN SR s
            ON d.relationID = d.SRID
        LEFT JOIN Events e
            ON d.relationID = e.eventID

回答2:

Something along these lines

SELECT 
    CASE WHEN SR.DocumentType = 'SR' THEN s.SRid ELSE e.eventId END AS Id
FROM documents d
LEFT JOIN SR s
ON s.SRId = d.relationId
LEFT JOIN Events e
ON e.EventId = d.relationId

回答3:

This should work.

Select * 
from documents d
    left join SR s on d.relationid = d.SRId
    left join Events e on d.relationid = e.eventid
where d.relationid = ( case when d.documenttype = 'SR' then s.SRId else e.eventid end)

回答4:

You no need to write Where clause as you need all records

Below Query Works for sure

Select DocumentID,
       DocumentType,
       RelationID,
       case when DocumentType = 'SR' then S.SRID else E.EventID end as Doc_DataID
From 
      Documents D
Left outer join SR S on D.RelationID = S.SRID
LEFT OUTER JOIN EVENTS E on E.EventiD = D.RelationID