Show only top n records and add '...' FOR XML - sql

Question

I want to list only a few records (say 3) and adds ... if there are more in a single line using FOR XML PATH('').
So far I have written
;WITH SRC AS (
select 'A' grp, 'abc' rec
union select 'A', 'def'
union select 'A', 'ghi'
union select 'A', 'jkl'
union select 'B', 'mno'
)
SELECT (
SELECT STUFF((
SELECT TOP 3 ',' + rec FROM SRC
WHERE SRC.grp = tableA.grp
FOR XML PATH('')
), 1, 1, '') + CASE WHEN (SELECT COUNT(1) FROM SRC WHERE SRC.grp = tableA.grp) > 3 THEN ',...' ELSE '' END
)
FROM (SELECT 'A' grp) tableA
The above works but I want to know if there is a way not to select from SRC twice to accomplish the job (one for the data, one for the count), because the where clause could be a big one in some case.
I can't move the where clause into the CTE as well because the condition depends on the result of another select (e.g. the select from tableA).
Using MS SQL SERVER 2008 R2
Thank you

Answer 1

Create a XML using the first 4 rows for the group. Shred the first three nodes and return ... for the fourth row.
with SRC as
(
select 'A' grp, 'abc' rec
union select 'A', 'def'
union select 'A', 'ghi'
union select 'A', 'jkl'
union select 'B', 'mno'
union select 'B', 'pqr'
)
select (
select ','+S2.X.value('.', 'nvarchar(max)')
from (
select top(4) S.rec, '...' as eli
from SRC as S
where S.grp = tableA.grp
for xml path('X'), type
-- order by ?
) as S1(X)
cross apply S1.X.nodes('(X[position() lt 4]/rec, X[position() eq 4]/eli)/text()') as S2(X)
for xml path(''), type
).value('substring(text()[1], 2)', 'nvarchar(max)')
from (select 'A') as tableA(grp);
How does this work?
The innermost query creates an XML using the first four rows.
select top(4) S.rec, '...' as eli
from SRC as S
where S.grp = tableA.grp
for xml path('X'), type
-- order by ?
that looks like this
<X>
<rec>abc</rec>
<eli>...</eli>
</X>
<X>
<rec>def</rec>
<eli>...</eli>
</X>
<X>
<rec>ghi</rec>
<eli>...</eli>
</X>
<X>
<rec>jkl</rec>
<eli>...</eli>
</X>
The XML is then shredded using nodes()
S1.X.nodes('(X[position() lt 4]/rec, X[position() eq 4]/eli)/text()') as S2(X)
X[position() lt 4]/rec gives you the rec value from the first three Xnodes and X[position() eq 4]/eli gives you the value from eli for the fourth row.
The result of the shredded XML as a table.
abc
def
ghi
...
Then comes a regular XML concatenation using for xml path with the little twist that I use substring(text()[1], 2) in a values() clause to remove the first comma.
Another way:
You can use row_number() in a case to figure out when the fourth row i returned a. Note that I have add an order by clause both to the row_number() function and the nested query to make sure what rows are returned.
with SRC as
(
select 'A' grp, 'abc' rec
union select 'A', 'def'
union select 'A', 'ghi'
union select 'A', 'jkl'
union select 'B', 'mno'
union select 'B', 'pqr'
)
select (
select top(4) ','+case when row_number() over(order by S.rec) = 4 then '...' else S.rec end
from SRC as S
where S.grp = tableA.grp
order by S.rec
for xml path(''), type
).value('substring(text()[1], 2)', 'nvarchar(max)')
from (select 'A') as tableA(grp);