Concatenate an arbitrary length of columns per row - sql-server

Question

How can I concatenate an arbitrary length of columns per row? I tried the following, but the function as is requires one to specify the column names:
SELECT CONCAT([C1], [C2], ...) FROM [dbo.table];
How can I achieve the same result without specifying each column name explicitly?

Answer 1

You'd need to use dynamic SQL. You can query the system catalg view sys.columns to get the column names, and then use SQL Server's XML Extension to concatenate the rows to a single string giving your final SQL to execute:
DECLARE #TableName SYSNAME = 'dbo.YourTable';
DECLARE #SQL NVARCHAR(MAX) = 'SELECT CONCAT(' +
STUFF(( SELECT ',' + QUOTENAME(c.Name)
FROM sys.columns c
WHERE [object_id] = OBJECT_ID(#TableName)
FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)'), 1, 1, '') + ')
FROM ' + #TableName + ';';
EXECUTE sp_executesql #SQL;
ADDENDUM
If you want to delimit your columns, you can add a further concatenation while you are creating your column list:
DECLARE #TableName SYSNAME = 'dbo.YourTable',
#Delimiter VARCHAR(10) = ', ';
DECLARE #SQL NVARCHAR(MAX) = 'SELECT CONCAT(' +
STUFF(( SELECT ',''' + #Delimiter + ''',' + QUOTENAME(c.Name)
FROM sys.columns c
WHERE [object_id] = OBJECT_ID(#TableName)
FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)'), 1, LEN(#Delimiter) + 5, '') + ')
FROM ' + #TableName + ';';
EXECUTE sp_executesql #SQL;
ADDENDUM 2
To avoid the delimiter being added when the value is null, e.g instead of ending up with:
1,,,2,3
You simply get
1,2,3
You need to slightly amend the logic, before it was generating a query that was like:
CONCAT([C1], ',', [C2], ',', [C3])
Instead you want:
CONCAT([C1], ',' + [C2], ',' + [C3])
Because you are now using ',' + [C2] if [C2] is null, the result will be null, so the delimiter will be removed:
DECLARE #TableName SYSNAME = 'dbo.YourTable',
#Delimiter VARCHAR(10) = ', ';
DECLARE #SQL NVARCHAR(MAX) = 'SELECT CONCAT(' +
STUFF(( SELECT ',''' + #Delimiter + ''' + ' + QUOTENAME(c.Name)
FROM sys.columns c
WHERE [object_id] = OBJECT_ID(#TableName)
FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)'), 1, LEN(#Delimiter) + 7, '') + ')
FROM ' + #TableName + ';';
EXECUTE sp_executesql #SQL;
ADDENDUM 3
To remove the first column you can use ROW_NUMBER() on the sys.columns system catalog view, then exclude the first column:
DECLARE #TableName SYSNAME = 'dbo.YourTable',
#Delimiter VARCHAR(10) = ', ';
DECLARE #SQL NVARCHAR(MAX) = 'SELECT CONCAT(' +
STUFF(( SELECT ',''' + #Delimiter + ''' + ' + QUOTENAME(c.Name)
FROM ( SELECT name,
RowNumber = ROW_NUMBER() OVER(ORDER BY column_id)
FROM sys.columns c
WHERE [object_id] = OBJECT_ID(#TableName)
) AS c
WHERE c.RowNumber != 1 -- not first column
FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)'), 1, LEN(#Delimiter) + 7, '') + ')
FROM ' + #TableName + ';';
EXECUTE sp_executesql #SQL;

Answer 2

You need to use Dynamic-SQL for this:
Warning:
I've used tempdb (tempdb.sys.columns) because I cannot create normal tables in demo. In your case use your normal database. And change condition to: WHERE object_id = OBJECT_ID('table_name').
LiveDemo
CREATE TABLE #tab(ID INT, C1 INT, C2 INT, C3 INT);
INSERT INTO #tab VALUES (1, 1,2,3), (2, 2,3,4);
DECLARE #cols NVARCHAR(MAX);
SET #cols = STUFF(
(SELECT ',' + QUOTENAME(name)
FROM tempdb.sys.columns
WHERE
object_id = (SELECT object_id
FROM tempdb.sys.objects
WHERE NAME like '#tab%' AND Type = 'U')
AND name LIKE 'C%'
ORDER BY column_id
FOR XML PATH(''), TYPE
).value('.', 'NVARCHAR(MAX)')
,1,1,'');
DECLARE #query NVARCHAR(MAX)=
N'SELECT ID, CONCAT(<placeholder>) AS concated_columns FROM #tab';
SET #query = REPLACE(#query, '<placeholder>', #cols);
EXEC [dbo].[sp_executesql]
#query;
EDIT:
If you need specific character between concatenated values use:
(SELECT ',' + CONCAT(QUOTENAME(name) , ','' ''' )
LiveDemo2

Answer 3

To concat all columns in arbitrary table:
DECLARE #Columns nvarchar(MAX)
SELECT #Columns = ISNULL(#Columns + ',','') + COLUMN_NAME
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = 'TableName' AND TABLE_SCHEMA='dbo'
DECLARE #sql nvarchar(MAX)
SET #sql = N'SELECT CONCAT('+#Columns+')
FROM dbo.TableName'
EXEC sp_executesql #sql

Answer 4

Use dynamic SQL and remember to cater for nulls!
declare #sql nvarchar(max), #t sysname, #c sysname
select #sql = 'select ', #t = '[dbo].[CONTACTS]' /* <---- YOUR TABLE NAME HERE */
declare cols cursor for
select name from sys.columns where object_id = object_id(#t) order by column_id
open cols
fetch next from cols INTO #c
while ##FETCH_STATUS = 0
begin
select #sql = #sql + 'convert(nvarchar(max), isnull(' + #c + ', '''')) + '
fetch next from cols INTO #c
end
close cols
deallocate cols
select #sql = left(#sql, len(#sql)-2) + ' from ' + #t
exec sp_executesql #sql

Answer 5

If your table has a primary key column, you could use a correlated subquery like the example below. SqlFiddle here.
SELECT (
( SELECT *
FROM dbo.table1 AS t2
WHERE t2.C1 = t1.C1 --specify primary key column(s) here
FOR
XML PATH('test')
, TYPE
)).value('/test[1]', 'nvarchar(MAX)') AS ConcatenatedValue
FROM dbo.table1 AS t1;