How to update DB2 table with a join?

问题:

I have two tables and I want to update by joining them. I am using DB2 V9.7.

ORDER_APPROVALS

ORDER_ID    CREATED_BY_ID   CREATED_BY_NAME PROCESS_DT           
-------------------------------------------------------
234         2               admin           (null)    
307         2               admin           (null)    
313         2               admin           11-11-2013    

ORDER_ATTRIBUTE

ORDER_ID    ATTRIBUTE_ID    VALUE
-----------------------------------
234         123             ? --(ORDER_APPROVALS.CREATED_BY_NAME)
307         123             ? --(ORDER_APPROVALS.CREATED_BY_NAME)

I want to update value field against Attribute_ID 123. So far I tried following query. But that does not work. I have tried similar kind of join in Netezza and that works. Want to know, how to do it in DB2?

update ORDER_ATTRIBUTE OT set OT.VALUE = 
(select CREATED_BY_NAME from ORDER_APPROVALS OA 
where OA.ORDER_ID = OT.ORDER_ID and OA.PROCESS_DT is NULL) 
where OT.ATTRIBUTE_ID = 123 and OT.ORDER_ID in 
(select ORDER_ID from ORDER_APPROVALS where PROCESS_DT is NULL)

回答1:

You are looking for the MERGE statement:

merge into ORDER_ATTRIBUTE ot
    using (select ORDER_ID, CREATED_BY_NAME
             from ORDER_APPROVALS
            where PROCESS_DT is null) oa
    on
       (ot.ORDER_ID = oa.ORDER_ID)
    when matched 
     and ot.ATTRIBUTE_ID = 123
    then update set VALUE = oa.CREATED_BY_NAME;

回答2:

I think you need to use a derived table to accomplish this:

update order_attributes
set value = (
  select
  created_by_name
  from(    
    select
    created_by_name,
    oa.order_id
    from
    order_approvals oa
    left outer join order_attributes ot
    on oa.order_id = ot.order_id
    AND OT.ATTRIBUTE_ID = 123 
    and OT.ORDER_ID in 
      (select ORDER_ID from ORDER_APPROVALS where PROCESS_DT is NULL)
    ) ORDERS
 WHERE orders.order_id = order_attributes.order_Id
    )

回答3:

update ORDER_ATTRIBUTE OT set OT.VALUE = 
(select CREATED_BY_NAME from ORDER_APPROVALS OA 
where OA.ORDER_ID = OT.ORDER_ID and OA.PROCESS_DT is NULL) 
where OT.ATTRIBUTE_ID = 123 

With this it's enough.