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dm  mc  ssdm ssmc
01  a 
02  b
03  c  01
04  d  02
05  e  03

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dm  mc  ssdm ssmc
01  a 
02  b
03  c  01  a
04  d  02  b
05  e  03  c

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SQL code:
update t
set ssmc = (
select mc
from t t2
where
t.dm = t2.ssdm
);

try һϡ£

---

SQL code:
update t5
set ssmc=(
select t2.val1
from t5 t2
where t2.dm=t5.ssdm
);

---

update t5
set ssmc=(
select t2.mc
from t5 t2
where t2.dm=t5.ssdm
);

---

update ta a set
ssmc=(select mc from ta b where a.dm=b.ssdm)
where exists(select 1 from ta b where a.dm=b.ssdm)

---

ÒýÓÃ

update t5
set ssmc=(
select t2.mc
from t5 t2
where t2.dm=t5.ssdm
);

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dm  mc  ssdm ssmc
01  a 
02