SQL - Prepend a value if missing - sql

Question

Code/data:
DECLARE #T TABLE
(
[Col1] VARCHAR(20)
, [RowNum] INT
) ;
INSERT INTO #T
VALUES
( N'second', 1 )
, ( N'fifth', 4 )
, ( N'fourth', 3 )
--, ( N'zzz', 1 )
, ( N'third', 2 )
---- OR when "zzz" is part of this list
--VALUES
-- ( N'second', 2 )
-- , ( N'fifth', 5 )
-- , ( N'fourth', 4 )
-- , ( N'zzz', 1 )
-- , ( N'third', 3 )
SELECT STUFF ((
SELECT ',' + [SQ].[Col1]
FROM
(
SELECT N'zzz' AS [Col1]
, 1 AS [RowNum]
UNION
SELECT [Col1]
, [RowNum]
FROM #T
) AS [SQ]
FOR XML PATH ( '' ), TYPE
).[value] ( '.', 'varchar(MAX)' ), 1, 1, ''
) ;
Current output:
fifth,fourth,second,third,zzz
Goal:
Prepend "zzz," in the output string if missing in the 2nd part of the union AND the values should be in ASC ordered based on the values specified in [rownum] field defined in the 2nd part of the union. If "zzz" exists in the 2nd part of the input already (it will always be RowNum 1 in that case), it should return it only once as the first value.
Expected output:
zzz,second,third,fourth,fifth
UPDATED the requirement due to an error on my part when creating this post. Updated code/data represents more accurate scenario. Please note the RowNum seq in the 2nd part of the UNION, it also starts with 1, but this time, it might or might not be associated to "zzz" Basically, I want to prepend "zzz" in the comma-delimited & ordered output if it doesn't exist.

Answer 1

Hope the below one will help you.
SELECT ',' + [SQ].[Col1]
FROM
(
SELECT N'first' AS [Col1],1 AS [RowNum]
UNION
SELECT [ABC].[Col1],[ABC].[RowNum]
FROM
(
VALUES
( N'second', 2 )
, ( N'fifth', 5 )
, ( N'fourth', 4 )
--, ( N'first', 1 )
, ( N'third', 3 )
) AS [ABC] ( [Col1], [RowNum] )
) AS [SQ]
ORDER BY [RowNum]
FOR XML PATH ( '' ), TYPE
).[value] ( '.', 'varchar(MAX)' ), 1, 1, ''
) ;
Returns an output
first,second,third,fourth,fifth

Answer 2

Attached the Answer for the updated Scenario-
DECLARE #T TABLE
(
[Col1] VARCHAR(20)
, [RowNum] INT
) ;
INSERT INTO #T
VALUES
( N'second', 1 )
, ( N'fifth', 4 )
, ( N'fourth', 3 )
--, ( N'zzz', 1 )
, ( N'third', 2 )
---- OR when "zzz" is part of this list
--VALUES
-- ( N'second', 2 )
-- , ( N'fifth', 5 )
-- , ( N'fourth', 4 )
-- , ( N'zzz', 1 )
-- , ( N'third', 3 )
SELECT STUFF ((
SELECT ',' + [SQ].[Col1]
FROM
(
SELECT N'zzz' AS [Col1]
, 0 AS [RowNum]
UNION
SELECT [Col1]
, [RowNum]
FROM #T
) AS [SQ]
ORDER BY [RowNum]
FOR XML PATH ( '' ), TYPE
).[value] ( '.', 'varchar(MAX)' ), 1, 1, ''
) ;
Returns
zzz,second,third,fourth,fifth

Answer 3

Common Table Expressions (CTEs) provide a handy way of breaking queries down into simpler steps. Note that you can view the results of each step by switching out the last select statement.
with
Assortment as (
-- Start with the "input" rows.
select Col1, RowNum
from ( values ( N'second', 2 ), ( N'fifth', 5 ), ( N'fourth', 4 ),
-- ( N'first', 1 ),
( N'third', 3 ) ) ABC ( Col1, RowNum ) ),
ExtendedAssortment as (
-- Conditionally add "first".
select Col1, RowNum
from Assortment
union all -- Do not remove duplicate rows.
select N'first', 1
where not exists ( select 42 from Assortment where Col1 = N'first' ) )
-- Output the result.
-- Intermediate results may be seen by uncommenting one of the alternate select statements.
-- select * from Assortment;
-- select * from ExtendedAssortment;
select Stuff(
( select N',' + Col1 from ExtendedAssortment order by RowNum for XML path(N''), type).value( N'.[1]', 'NVarChar(max)' ),
1, 1, N'' ) as List;
The same logic can be performed using tables for input:
-- Rows to be included in the comma delimited string.
declare #Input as Table ( Col1 NVarChar(20), RowNum Int );
insert into #Input ( Col1, RowNum ) values
( N'second', 2 ), ( N'fifth', 5 ),
--( N'ZZZ', 17 ), -- Test row.
( N'fourth', 4 ), ( N'third', 3 );
select * from #Input;
-- Mandatory value that must appear in the result. One row only.
declare #Mandatory as Table ( Col1 NVarChar(20), RowNum Int );
-- By using the maximum negative value for an Int this value will be prepended
-- (unless other rows happen to have the same RowNum value).
insert into #Mandatory ( Col1, RowNum ) values ( N'ZZZ', -2147483648 );
select * from #Mandatory;
-- Process the data.
with
AllRows as (
select Col1, RowNum
from #Input
union all
select Col1, RowNum
from #Mandatory
where not exists ( select 42 from #Mandatory as M inner join #Input as I on M.Col1 = I.Col1 ) )
-- Output the result.
-- Intermediate results may be seen by uncommenting the alternate select statement.
--select * from AllRows;
select Stuff(
( select N',' + Col1 from AllRows order by RowNum for XML path(N''), type).value( N'.[1]', 'NVarChar(max)' ),
1, 1, N'' ) as List;