Use the .WRITE (expression, @Offset,@Length) clause to perform a partial or full update of varchar(max), nvarchar(max), and varbinary(max) data types.
For example, a partial update of a varchar(max) column might delete or modify only the first 200 characters of the column, whereas a full update would delete or modify all the data in the column. .WRITE updates that insert or append new data are minimally logged if the database recovery model is set to bulk-logged or simple.
SET column_name.WRITE (expression, @offset, @length)
expression is the value that is copied to column_name. expression must evaluate to or be able to be implicitly cast to the column_name type. If expression is set to NULL, @Length is ignored, and the value in column_nameis truncated at the specified @Offset.
@Offset is the starting point in the value of column_name at which expression is written. @Offset is a zero-based ordinal position, is bigint, and cannot be a negative number. If @Offset is NULL, the update operation appends expression at the end of the existing column_name value and @Length is ignored. If @Offset is greater than the length of the column_name value, the Database Engine returns an error. If @Offset plus @Length exceeds the end of the underlying value in the column, the deletion occurs up to the last character of the value. If @Offset plus LEN(expression) is greater than the underlying declared size, an error is raised.
@Length is the length of the section in the column, starting from @Offset, that is replaced by expression. @Length is bigint and cannot be a negative number. If @Length is NULL, the update operation removes all data from @Offset to the end of the column_name value.
The regular update statement results in overwriting the entire string using full logging and it’s very inefficient when dealing with large value updates.
Note: The update using WRITE method will fail if the value is null.