krishnaroopa (7/1/2010)
Also, why not it work like this:'-' being string,
other two can implicitly convert to string?
They can, but they won't. SQL Server uses well-defined rules for data type precedence for implicit conversions. Lower predence data types will always convert to higher precedence data types; never the other way around.
These rules are documented in Books Online. Use the index to find "data types [SQL Server]], precedence". Or read through the discussion, I believe I posted a link to this subject in the online version of BOL on one of the first pages.