原始查询 - MYSQL（IF ELSE里面的sql）

Im trying to execute a query that collects all of the data.
So here is my condition if the client_type of a row is Corporation the query will fetch the data of corporation field and it will assign it to fullname or else it will fetch the firstname, lastname and middlename and assign it as fullname. I manage to do this query. but i have an error.

  (SELECT SA.id,SA.sa_code,S.importer_name as name,
  CONCAT_WS(" ",(IF(C.client_type = 'Corporation',C.corporation), 
  C.firstname,' ',C.middlename,' ',C.lastname)) as fullname,
  C.client_type,C.corporation,
  CI.invoice_code,CI.created_at as invoice_date,
  SA.created_at as salesagreement_date,
  S.id as seller_id, C.id as 
  buyer_id,SA.product_id,P.unit_code,SA.purchase_price
  FROM sales_agreement SA
  LEFT JOIN charge_invoice CI ON CI.id = SA.invoice_id
  LEFT JOIN importer S ON S.id = SA.seller_id
  LEFT JOIN clients C ON C.id = SA.buyer_id
  LEFT JOIN products P ON P.id = SA.product_id
  WHERE SA.deleted = 0
  ORDER BY SA.created_at DESC
  )P

For the sake of clarity, I would replace your call to CONCAT_WS with an explicit CASE expression:

SELECT
    SA.id,
    ...
    CASE WHEN C.client_type = 'Corporation'
         THEN C.corporation
         ELSE CONCAT_WS(' ', C.firstname, C.middlename, C.lastname) END AS fullname,
    ...
FROM sales_agreement SA
...

The problem with your current call to CONCAT_WS is that the IF function call does not have an else condition, and also you have a list of things wrapped in parentheses being passed as the first (and only) parameter to CONCAT_WS. In addition, if you pass a space separator to CONCAT_WS, then you don't have to also include spaces in the call itself. This is the whole point of CONCAT_WS that it handles the separator for you.