Gett all column names of table Target_Table with comma separated (,) by commas

DECLARE @SQL    nvarchar(4000) = ''

SELECT @SQL += '['+COLUMN_NAME+']' + ', ' FROM INFORMATION_SCHEMA.COLUMNS

WHERE TABLE_NAME = 'Employee'

select SUBSTRING(@SQL,0,len(@SQL))

result-

[EmployeeID], [OldID], [Contract_Type], [EmployeeCode]