Concatenation of strings by for xml path

问题:

Hi! Today I learned for xml path technique to concatenate strings in mssql. Since I've never worked with xml in mssql and google hasn't helped, I need to ask you.

Let's imagine the default case. We need to concatenate some strings from a table:

declare @xmlRepNames xml = (
    select          
        ', [' + report_name + ']'
    from (
        select distinct
            report_order,
            report_name
        from #report
    ) x
    order by
        report_order
    for xml path(''), type)

select
    stuff((select @xmlRepNames.value('.', 'nvarchar(max)')), 1, 1, '')

So I get smth like this:

[str1], [str2], [strn]

Ok. It works fine. But I have two very similar concatenate blocks. The difference is just in the way the result string looks like:

• [str1], [str2], [strn]
  and
• isnull([str1], 0) as [str1], isnull([str2], 0) as [str2], isnull([strn], 0) as [strn]

So I can write 2 very similar code blocks (already done, btw) with different string constructors or to try extend previous code to has xml variable containing 2 kind of constructors and then concatenate by xml node type. Doing 2nd way I met some problems - I wrote this:

declare @xmlRepNames xml = (
    select
        ', [' + report_name + ']' as name,
        ', isnull([' + report_name + '], 0) as [' + report_name + ']' as res
    from (
        select distinct
            report_order,
            report_name
        from #report
    ) x
    order by
        report_order
    for xml path(''), type)

select
    stuff((select @xmlRepNames.value('/name', 'nvarchar(max)')), 1, 1, ''),
    stuff((select @xmlRepNames.value('/res', 'nvarchar(max)')), 1, 1, '')

but it raise error "XQuery [value()]: 'value()' requires a singleton (or empty sequence), found operand of type 'xdt:untypedAtomic *'".
To replace, e.g., '/name' to '/name[1]' or any other '/name[x]', will return just x-th 'name' record but not all 'name' records concatenated.
[question]: is it possible to solve problem 2nd way like I want and if it's possible then how?
[disclaimer]: the problem isn't really serious for me now (1st way just a little bit uglier but also fine), but it seems very interesting how to come over :) Thanks!

回答1:

Your subquery cannot return two values. If you just want to concatenate strings, you do not need the xml data type at all. You can do the stuff() and subquery in a single statement:

declare @Rep1Names nvarchar(max) = (
    stuff((select ', [' + report_name + ']' as name
           from (select distinct report_order, report_name
                 from #report
                ) x
           order by report_order
           for xml path('')
          )
         ), 1, 1, '');

declare @Rep2Names nvarchar(max) = (
    stuff(select ', isnull([' + report_name + '], 0) as [' + report_name + ']' as res
           from (select distinct report_order, report_name
                 from #report
                ) x
           order by report_order
           for xml path('')
          )
   ), 1, 1, '');

回答2:

Ok, so I haven't been completely satisfied the way Gordon Linoff suggested and since I've found this kind of problem actual for me I'm adding here another solution without using for xml path:

declare
    @pivot_sequence nvarchar(max),
    @columns_sequence nvarchar(max)

select
    @pivot_sequence = coalesce(@pivot_sequence + ', [', '[') 
        + col_name + ']',
    @columns_sequence = coalesce(@columns_sequence + ', ', '')
        + 'isnull([' + col_name + '], 0) as [' + col_name + ']'
from some_table
order by
    /* some_columns if needed to order concatenation */

Obviously, it'll work much slower but in cases when you haven't many rows it won't drastically affect on performance. In my case I have dynamic pivot query and these strings are built for it - I won't have many columns in pivot so it's fine for me.