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
);

我要获取附加到实体的文件和文件夹列表。尝试了下面的查询，但是如果文件夹或文件中都没有记录，它会给出空记录

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

样本数据

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