Update between two tables when there is no relation between them

I needed a SQL statement which fills the null values from the second column of #T1 table with values from #T2(C1).

There is no foreign key or match between the columns of those two tables.

I found two approaches:

1. Using CTE

 create table #T1 (C1 varchar(10), T2C1 varchar(10))
 create table #T2 (C1 varchar(10))

 insert into #T1 values ('A1', '1')
 insert into #T1 values ('A2', null)
 insert into #T1 values ('A3', null)
 insert into #T1 values ('A4', '4')
 insert into #T1 values ('A5', null)

 insert into #T2 values ('a')
 insert into #T2 values ('b')

 ;with t2 as
 (
 select C1, row_number() over (order by C1) as Index2
 from #T2
 )
 ,t1 as
 (
 select T2C1, row_number() over (order by C1) as Index1   
 from #T1
 where T2C1 is null
 )
 update t1
 set t1.T2C1 = t2.C1
 from t2
 where t1.Index1 = t2.Index2

 select * from #T1

 drop table #T1
 drop table #T2

2. With Derived Tables

 create table #T1 (C1 varchar(10), T2C1 varchar(10))
 create table #T2 (C1 varchar(10))

 insert into #T1 values ('A1', '1')
 insert into #T1 values ('A2', null)
 insert into #T1 values ('A3', null)
 insert into #T1 values ('A4', '4')
 insert into #T1 values ('A5', null)

 insert into #T2 values ('a')
 insert into #T2 values ('b')

 update #T1
 set T2C1 = cj.C1
 from #T1
 join (select T2C1, row_number() over (order by C1) as Index1, C1
    from #T1
    where T2C1 is null) ci on ci.C1 = #T1.C1
 join (select C1, row_number() over (order by C1) as Index2
   from #T2) cj on ci.Index1 = cj.Index2

 select * from #T1

 drop table #T1
 drop table #T2

Update
3.From SO, with table variables

create table #T1 (C1 varchar(10), T2C1 varchar(10))
create table #T2 (C1 varchar(10))

insert into #T1 values ('A1', '1')
insert into #T1 values ('A2', null)
insert into #T1 values ('A3', null)
insert into #T1 values ('A4', '4')
insert into #T1 values ('A5', null)

insert into #T2 values ('a')
insert into #T2 values ('b')


DECLARE @Target TABLE
(
     MyId INT IDENTITY(1,1) PRIMARY KEY
    ,T1_pk varchar(10) NOT NULL UNIQUE
);
INSERT  @Target (T1_pk)
SELECT  C1
FROM    #T1 
WHERE   T2C1 IS NULL;

DECLARE @Source TABLE
(
     MyId INT IDENTITY(1,1) PRIMARY KEY
    ,C1 VARCHAR(10) NOT NULL
);
INSERT  @Source (C1)
SELECT  C1
FROM    #T2;

UPDATE  #T1
SET     T2C1 = src.C1
FROM    #T1 t
INNER JOIN @Target trg ON t.C1 = trg.T1_pk 
INNER JOIN @Source src ON trg.MyId = src.MyId;

select * from #T1

drop table #T1
drop table #T2