>I have a Prefix tables containing records with a Prefix field such as "AB", "KN", "LL" and "BB" as well as a Tag field such as "CM111", "CM222", "CM333" and "
>"CM444". I process a main table having a title such as "Abertimer". For each record of that table, I need to find the record in teh Prefix table so I can extract the Tag value. For example, in this case, it would be the one having a Prefix field "AB". This is because "Abertimer" starts with "AB". I am not sure about the syntax to use. The reason I ask is that I also have Prefix values having four letters. So, I cannot compare with the first two letters only. This is variable. Anyone would have an idea on how to do this?

Try

SELECT TOP (1) p.* FROM Prefixes p
    JOIN Master m ON m.title LIKE p.Prefix + '%'
  ORDER BY LEN(p.Prefix) DESC