Update value of one record with the value of another record in the same table

Question

I've got a tough SQL question for you guys. I am working with the Webform module of drupal.

How this module works:
- every item on the webform is a new record in the 'webform_submitted_data'-table,
- sid = registered user
- cid = webform field

Structure of the table:

NID | SID | CID | NO | DATA

1 --- 168 --- 1 --- 0 --- XXX

1 --- 168 --- 2 --- 0 --- YYY

The problem: - CID 64 should get updated with the value of CID 56 of the same SID.

Answer 1

I think it should be as follows:

UPDATE webform_submitted_data as a, webform_submitted_data as b 
SET a.data = b.data WHERE a.sid = b.sid AND a.cid = 64 AND b.cid = 56

Basically, you self-join the table (matching SIDs) and then you have both fields in your query, so you can simply refer to each at once.

Answer 2

This would copy the Data of CID 56 to CID 64, for each value of SID. It's SQL Server syntax.

update  yt64
set     Data = yt56.Data
from    YourTable yt64
join    YourTable yt56
on      yt64.SID = yt56.SID
where   yt64.CID = 64
        and yt56.CID = 56