You have to give a table variable an alias in order to do this kind of thing.

DECLARE @MyTable TABLE

(

COl1 INT,

COl2 INT

);

SELECT MT.Col1 FROM @MyTable AS MT;

UPDATE MT

SET Co2 = A.Cik2

FROM @MyTable AS MT

INNER JOIN Table2 AS A

WHERE A.Col1 = MT.Col1;

Even better yet, on the Update From, use Merge instead.

MERGE INTO @MyTable AS Tgt

USING (SELECT Cik2, Col1 FROM dbo.Table2) AS Src

ON Tgt.Col1 = Src.Col1

WHEN MATCHED THEN UPDATE

SET Col1 = Src.Col1;

Merge has a few advantages over Update From, in terms of ACID-compliance in the update. It's also ISO compliant, if that matters to you, where Update From is T-SQL proprietary. But the ACID properties, like Update From can have multiple, conflicting updates to the same row, while Merge can't, are what really matter.