Make multiple columns into one column in sql - sql

Question

I have a table Employee which have several fields like FirstName,LastName,Email,....... . So what i want to do is that in my case selection of column is dynamic
Declare #columnNeeded nvarchar(max)
Example one
Set #columnNeeded = 'FirstName,Email'
Select #columnNeeded from Employee
Example Two
Set #columnNeeded = 'FirstName,LastName'
Select #columnNeeded from Employee
This is pretty simple , now what i want is that regardless of what column will be in result set i need all column selected in one column as comma seperated string . I saw Group_Concat() in mysql but don't know how to do this in sql .So is this possible ?

Answer 1

You can do this with dynamic SQL:
declare #sql nvarchar(max) = 'select #columns from Employee';
declare #columnNeeded nvarchar(max) = 'FirstName,Email';
set #sql = replace(#sql, '#columns', #columnNeeded);
exec sp_executesql #sql;
EDIT:
If you want them as one column, you could do:
declare #sql nvarchar(max) = 'select #columns from Employee';
declare #columnNeeded nvarchar(max) = 'FirstName,Email';
set #sql = replace(replace(#sql, '#columns', #columnNeeded), ',', '+'',''+');
exec sp_executesql #sql;
To type-safe you would cast the column values:
declare #tmp nvarchar(4000) = 'cast(' +
replace(#columnNeeded, ',', ', nvarchar(4000)), cast(') +
', nvarchar(4000))'
set #sql = replace(replace(#sql, '#columns', #columnNeeded), ',', '+'',''+');
If this works as expected, it adds cast(<col> as nvarchar(4000)) to each of the columns in the list.

Answer 2

You have to use Dynamic SQL. Since you have different Data types in your table you may have to convert the columns to Varchar to concatenate the result into single column.
DECLARE #sql NVARCHAR(max),
#cols NVARCHAR(max) ='FirstName,Email'
SELECT #cols = 'convert(varchar(100),'
+ Replace(#cols+')+', ',', ')+'',''+convert(varchar(100),')
SELECT #cols = LEFT(#cols, Len(#cols) - 1)
SET #sql ='select ' + #cols + ' from Employee '
--print #sql
EXEC Sp_executesql #sql;
Working Example :
CREATE TABLE #test1([Key] INT,ID INT,Value VARCHAR(2))
INSERT #test1
VALUES (1,1,'C' ),(2,1,'C' ),(3,1,'I' )
DECLARE #sql NVARCHAR(max),
#cols NVARCHAR(max) ='ID,Value'
SELECT #cols = 'convert(varchar(100),'
+ Replace(#cols+')+', ',', ')+'',''+convert(varchar(100),')
SELECT #cols = LEFT(#cols, Len(#cols) - 1)
SET #sql ='select ' + #cols + ' from #test1 '
EXEC Sp_executesql #sql;