left join error

[Garry Chen]

Hi All,
                Do you guys see any error in the following left join sql 
statement?  However,  the hive give me error as "Error: Error while compiling 
statement: FAILED: SemanticException [Error 10017]: Line 1:75 Both left and 
right aliases encountered in JOIN 'engine' (state=42000,code=10017)".  Please 
let me kow where I am doing wrong.  Thank you.

select date_key,ref, '' from master_request_tab left join dm_search_eng on 
substr(ref, 1, length(engine)) = engine where engine is null;