Drop multiple columns using subquery within alter table - sql-server

Question

I need to drop all but few columns from a table while I don't know number and names of these columns in advance. I want to get column names from sys.columns and then drop all columns except few. I need something like this:
alter table Table
drop column (
select a.name as ColumnName
from sys.all_columns a
INNER JOIN sys.tables b
on a.object_id = b.object_id
where b.name = 'Table'
where a.name NOT IN (
RowID,
RemoteUserId,
Email,
FirstName,
)
)

Answer 1

First You need to Create a Function to Split the Comma Separated Strings to Rows
CREATE FUNCTION [dbo].[FnSplit]
(
#List nvarchar(2000),
#SplitOn nvarchar(5)
)
RETURNS #RtnValue table
(
Id int identity(1,1),
Value nvarchar(100)
)
AS
BEGIN
While (Charindex(#SplitOn,#List)>0)
Begin
Insert Into #RtnValue (value)
Select
Value = ltrim(rtrim(Substring(#List,1,Charindex(#SplitOn,#List)-1)))
Set #List = Substring(#List,Charindex(#SplitOn,#List)+len(#SplitOn),len(#List))
End
Insert Into #RtnValue (Value)
Select Value = ltrim(rtrim(#List))
Return
END
GO
Then Create Below Stored Procedure
Create Procedure DropColumnExceptGivn
#table varchar(50),
#Column Varchar(max)
AS
BEGIN
SET NOCOUNT ON;
declare #variables AS TABLE (cols varchar(max))
declare #sql varchar(max) = ''
declare #sql1 varchar(max) = ''
declare #cols nvarchar(max) =''
set #sql1 = 'SELECT Stuff((select '',''+ Column_name '+'from INFORMATION_SCHEMA.COLUMNS where
table_name = ' +''''+#table+''''+
' and column_name not in (SELECT Value FROM dbo.FnSplit('+''''+#column+''''+','',''))
for xml path('''')),1,1,'''')'
print #sql1
insert into #variables
exec (#sql1)
select #cols = cols from #variables
select #cols
set #sql= 'alter table '+ #table + ' drop column ' + #cols
print #sql
exec (#sql)
end
go
--Creating a Sample Table
Create table pets1 (petid int, PetTypeID int, PetName Varchar(20),OwnerId int)
go
Then Execute the below Stored Procedure.
The Parameters are : 1. Table Name 2. The Columns need to be Left (Comma Seperated values)
exec DropColumnExceptGivn 'pets1','PetID, PetTypeID'
go
select *from pets1
Hope This will helps