MySQL外部连接与数据透视表

Question

CREATE TABLE link (
    entityid integer,
    fileid INT (11) NULL,
    folderid INT (11) NULL
);

CREATE TABLE file (
    fileid integer,
    name varchar
);    

CREATE TABLE folder (
    folderid integer,
    name varchar
);

I want to get list of files and folder attached to an entity. Have tried below query however it gives null records if there is no record in either folder or file

select fi.name,fo.name 
FROM link ll
LEFT OUTER JOIN file fi ON lk.FileId = fi.FileId  
LEFT OUTER JOIN folder fo ON lk.FileId = fo.FileId  
WHERE dl.entityid = 307

sample data

link        
entityid    fileid  folderid
22          1       null
22          2       null
22          3       null
22          4       null
22          null    33


file    
fileid  name
1       file1
2       file2
3       file3
4       file4

folder  
folderid    name
33         folder33
34         folder34
35         folder35
36         folder36

Answer 1

Just add the entityid to the select clause:

SELECT ll.entityid, fi.name, fo.name 
FROM link ll
LEFT OUTER JOIN file fi ON lk.FileId = fi.FileId  
LEFT OUTER JOIN folder fo ON lk.FileId = fo.FileId  
WHERE dl.entityid = 307;