CodeIgniter Concat

I've spent a few hours staring at this piece of code. Fresh eyes please! Here is a shortened version of the query:

You have an error in your SQL syntax; check the manual that corresponds to your MySQL 
server version for the right syntax to use near 
'FROM (`requests` c) JOIN `inventory` d ON `d`.`listing_seq_no` = `c' at line 7


SELECT DISTINCT `c`.`req_id`, `u`.`user_id`, `u`.`org_name`, 
CONCAT_WS(' ', `l`.`strength`, `l`.`unit)` as dos, `c`.`quantity` AS quantity1, 
    (SELECT sum(quantity) from inventory d2 
          WHERE d2.listing_seq_no = c.listing_seq_no 
    ) as inv_total,
 FROM (`requests` c)   
 JOIN `inventory` d 
    ON `d`.`listing_seq_no` = `c`.`listing_seq_no` 
 JOIN `listings` l 
    ON `l`.`listing_seq_no` = `c`.`listing_seq_no` 

EDIT: Original CodeIgniter Code snippet:

$this->db->select ( "c.req_id,
        u.user_id,开发者_开发问答
        u.org_name,
        l.tradename as name,
        CONCAT_WS(' ', l.strength, l.unit) as dos,
    );

This:

CONCAT_WS(' ', `l`.`strength`, `l`.`unit)`

Should be:

CONCAT_WS(' ', `l`.`strength`, `l`.`unit`)

try removing the parens around (requests c)

It's an old question, but you can use:

$this->db->select("<your_select_portion>", FALSE);

to avoid the auto-quoting feature of the CI DB class. I am a fan of using MySQL functions on select statements with CI, and this usually fixes the problem (except when you start using the query-caching feature, but that's another story).