UPDATE A 
   SET A.FDatabase = B.FDatabase
  FROM TClient AS A,
  (
SELECT FClientId,SUBSTRING(String,0,CHARINDEX(';',String)) AS FDatabase
FROM
(
SELECT FClientId,String = SUBSTRING(FConnectionString, CHARINDEX('Initial Catalog',FConnectionString) + LEN('Initial Catalog')+1,LEN(FConnectionString))
 FROM TClient WHERE FServerId = 106
 )AS T) AS B
WHERE A.FClientId = B.FClientId