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A±í
×ֶΠa1  a2  a3
      1  2  3
      6  2  7
B±í
×ֶΠb1  b2  b3
      4  5  3
      8  9  7
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sql = "Update A Set A.a1 = (select B.b3 from B,A where A.a3=B.b3 and A.a2='2') where A.a2='2'"
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SQL code:
UPDATE A SET A1=B1 from B WHERE A.A3=B.B3 AND A.A2=2

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SQL code:
update a
set a.a1=b.b1
from a,b
where a.a3=b.b3 and a.a2=2

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SQL code:

drop table a;
drop table b;

create table a(a1 int, a2 int, a3 int);
insert into a(a1,a2,a3)
select
1,2,3 union all select
6,2,7;

create table b(b1 int, b2 int, b3 int);
insert into b(b1,b2,b3)
select
4,5,3 union all select
8,9,7;

update a
set a1=b.b1
from a, b
where a.a3=b.b3 and a.a2=