TSQL: LEN vs DATALENGTH August 2, 2006
Often found in front of a computer, loom or sewing machine.
LEN defined in SQL Server Books online as Returns the number of characters, rather than the number of bytes, of the given string expression, excluding trailing blanks.
DATALENGTH defined as Returns the number of bytes used to represent any expression. DATALENGTH is especially useful with varchar, varbinary, text, image, nvarchar, and ntext data types because these data types can store variable-length data. The DATALENGTH of NULL is NULL.