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連接方式（推薦）
update a 
set a.name = b.name 
from #table1 a inner join #table2 b on b.id = a.id 
where a.name is null;

----或子查詢方式
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（推薦）
update #table1 
set #table1.name = b.name 
from #table2 b
where #table1.id = b.id and #table1.name is null;

--或子查詢方式
update #table1 
set name = ( select b.name from #table2 b where #table1.id = b.id ) 
where name is null;

merge #table1 a --要更新的目標表 
    using #table2 b --源表 
    on a.id = b.id and a.name is null--更新條件（即主鍵）
when matched --如果匹配，將源表指定列的值更新到目標表中 
    then update set a.name = b.name
when not matched 
    then insert values ( id, name ); --如果兩個條件都不匹配，將源表指定列的值插入到目標表中。此語句必須以分號結(jié)束

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

drop table #table1;
drop table #table2;