The code "does some work" where there is a matching dataid for the result of the subquery, but sets all other values to 0.

😎

USE tempdb;

GO

CREATE TABLE dbo.TableA

(

dataid int

,value int

)

INSERT INTO dbo.TableA(dataid,value) VALUES(1,1),(2,2),(3,3);

create table #tmpcopylibrary

(

dataid int,

value int

)

insert #tmpcopylibrary select 1,999;

SELECT * FROM dbo.TableA

UPDATE TableA

SET Value = isnull((select top 1 Value from #tmpcopylibrary

where dataid = TableA.dataid), 0)

SELECT * FROM dbo.TableA

DROP TABLE #tmpcopylibrary

DROP TABLE dbo.TableA

Results

Before

dataid value

----------- -----------

1 1

2 2

3 3

After

dataid value

----------- -----------

1 999

2 0

3 0

Somehow I believe that this isn't what you are after.