Trying to understanding what a SCALAR FUNCTION does

Question

I have been tasked with documenting the stored procedures and functions in our database and i have come across one which i am not sure what it is actually doing.

From looking at and googling parts of the code, i believe it is taking a XML string which is being passed to the function and merging it with the ID number. however i am not sure if this is correct

ALTER FUNCTION [dbo].[GetMergeItem]
(
@MergeData xml,
@MergeItemId nvarchar(max)
)
RETURNS nvarchar(max)
AS
BEGIN
DECLARE @MergeItemText nvarchar(max)

SELECT @MergeItemText = @MergeData.value('(/ArrayOfMergeFieldJson/MergeFieldJson[Id/text()=sql:variable("@MergeItemId")]/Text/text())[1]', 'nvarchar(max)') 
RETURN @MergeItemText
END

Can anyone tell me if what i think its doing is right and if not explain what it is actually doing?

Answer 1

This is the bit which I believe is causing your confusion.

SELECT @MergeItemText = @MergeData.value('(/ArrayOfMergeFieldJson/MergeFieldJson[Id/text()=sql:variable("@MergeItemId")]/Text/text())[1]', 'nvarchar(max)') 

What happens here is that the .value method returns a single value (hence scalar).

The first part of the brackets uses an xpath query to navigate to the relevant node in the xml structure. Part of the way down it looks like there are multiple attributes in the MergeFieldJson node, hence it begins with a square bracket to signify that a position is to be specified here. Within there the code then states what position to use (1 being the first position) using the passed in id value to continue pathing from. Then the xpath continues 2 further nodes from there and returns the first value found in the /text node.

the function then returns the value as nvarchar(max).