The differences between LEN and DATALENGTH in SQL Server

The differences between LEN and DATALENGTH in SQL Server

LEN
Returns the number of characters, rather than the number of bytes,
of the given string expression, excluding trailing blanks.

DATALENGTH
Returns the number of bytes used to represent any expression.

So what does that mean? It means that the LEN function will first right
trim the value and then give you a count of the charaters, the DATALENGTH
function on the other hand does not right trim the value and gives you the
storage space required for the characters.

Take a look at this example
declare @v nchar(5)
select @v ='ABC  '


select len(@v),datalength(@v)
The output for len is 3 while the output for datalength =10. The reason that
datalength returns the value 10 is because nvarchar uses 2 bytes to store 1
character by using unicode while varchar is using ascii which requires 1 byte
per charaters