How to use subselection result as temp table in MySQL update query?

Question

table A
    uid
    uname

temp table B
    uid
    uname

table B is not a real table but a result of subquery like

select uid, uname
from tableC left join tableD on tableC.pid = tableD.pid
where tableC.qid = tableD.qid
group by uid;

I want to update set A.uname = B.uname where A.uid = B.uid

how can I do that with MySQL?

Answer 1

Treat the subquery as a table in a JOIN :

UPDATE tableA AS a
JOIN (select uid, uname
    from tableC 
    left join tableD on tableC.pid = tableD.pid and tableC.qid = tableD.qid
    group by uid) AS b ON a.uid = b.uid
SET a.uname = b.uname

Also, note that in a LEFT JOIN , all the conditions that refer to the second table should be in the ON clause. Otherwise, you'll filter out all the non-matching rows, because the values of those columns will be NULL , and that will negate the point of using LEFT JOIN rather than INNER JOIN .

Answer 2

update a
set uname = B.name
from A a
inner join (
  select uname [name]
  from tableC left join tableD on tableC.pid = tableD.pid
  where tableC.qid = tableD.qid
  group by uid ) b
ON a.uid = b.uid