SQL基础：在SQL Server中使用子查询更新语句

create table #table1
( id int , name varchar(20) );
go

create table #table2
( id int , name varchar(20) );
go

insert into #table1 ( id, name ) values ( 1, ‘a’ ), ( 2, null ), ( 3, ‘c’ ), ( 4, ‘d’ ), ( 5, ‘e’ );
insert into #table2 ( id, name ) values ( 1, ‘a1’ ), ( 2, ‘b1’ ), ( 3, ‘c1’ );

—-join连接方式（推荐）
update a
set a.name = b.name
from #table1 a inner join #table2 b on b.id = a.id
where a.name is null;

—-或子查询方式
update a
set a.name = ( select b.name from #table2 b where a.id = b.id )
from #table1 a
where a.name is null;

—update … from（推荐）
update #table1
set #table1.name = b.name
from #table2 b
where #table1.id = b.id and #table1.name is null;

–或子查询方式
update #table1
set name = ( select b.name from #table2 b where #table1.id = b.id )
where name is null;

merge #table1 a –要更新的目标表
using #table2 b –源表
on a.id = b.id and a.name is null–更新条件（即主键）
when matched –如果匹配，将源表指定列的值更新到目标表中
then update set a.name = b.name
when not matched
then insert values ( id, name ); –如果两个条件都不匹配，将源表指定列的值插入到目标表中。此语句必须以分号结束

select * from #table1;
select * from #table2;

drop table #table1;
drop table #table2;