Question

I need 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.

Sample:

T1 (C1, T2C1)
A1, 1
A2, null
A3, null
A4, 4
A5, null
-------------
T2 (C1)
a
b

After update, the T1 will look like:

A1, 1
A2, a
A3, b
A4, 4
A5, null

I found two approaches:

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

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

My question is, can I achieve this without using windowing functions and with no cursors?

Update

@Damien_The_Unbeliever correctly points that to do this kind of update it is not possible without defining an ordering on tables, actually I think exactly said is without properly identify and link the rows from target table.

@Bogdan Sahlean has found another way, using table variables and IDENTITY column, which I’m happy with this solution, it’s another way
However, in the real application I will still use the windowing functions

Thanks all

Answer 1

1.I suppose you have a pk in target table (#T1).

2.Instead of ROW_NUMBER this solution uses IDENTITY(1,1) columns and two table variables.

3.I didn’t tested this solution.

DECLARE @t2_count INT = (SELECT COUNT(*) FROM #T2);

DECLARE @Target TABLE
(
     MyId INT IDENTITY(1,1) PRIMARY KEY
    ,T1_pk INT NOT NULL UNIQUE
);
INSERT  @Target (T1_pk)
SELECT  TOP(@t2_count) pk
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.pk = trg.T1_pk 
INNER JOIN @Source src ON trg.MyId = src.MyId;