Why does the query give me this error:

> 
> #1054 - Unknown column 'NOPL' in 'where clause' ?
> 
> 
> 

```
SELECT supplierprice.country, 
         supplierprice.networkname, 
         supplierprice.mcc, 
         supplierprice.mnc, 
         `288`.clientprice,
         `288`.ns, 
         client_list.currency
  FROM supplierprice
  INNER JOIN `288` 
  ON supplierprice.supp_price_id = `288`.net_id
  INNER JOIN client_list 
  ON `288`.clientid = client_list.clientid 
  WHERE `288`.ns <> `NOPL` ;

```