如何在MySQL更新查詢中將子選擇結果用作臨時表？

Question

table A
    uid
    uname

temp table B
    uid
    uname

表B不是真正的表，而是子查詢的結果，例如

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

我想更新集合A.uname = B.uname，其中A.uid = B.uid

我該如何使用MySQL？

Answer 1

將子查詢視為JOIN的表：

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

另外，請注意，在LEFT JOIN ，引用第二個表的所有條件都應在ON子句中。 否則，您將過濾掉所有不匹配的行，因為這些列的值將為NULL ，這將否定使用LEFT JOIN而不是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