DDL statements with variables for table and column names - sql-server

Question

In my stored procedure, I make a temp_tbl and want to add several columns in a cursor or while loop. All works fine the cursor (the creation of a temp_bl but I can´t add the column when the column string is in a varchar variable.
WHILE ##FETCH_STATUS = 0
BEGIN
SET #webadressenrow = 'Webadresse_'+CAST(#counter as nchar(10))
ALTER TABLE IVS.tmpBus
ADD #webadressenrow varchar(500) Null
fetch next from cur_web into #webadressen
SET #counter = #counter + 1
END
The code above results in a syntax error, while this code works:
WHILE ##FETCH_STATUS = 0
BEGIN
SET #webadressenrow = 'Webadresse_'+CAST(#counter as nchar(10))
ALTER TABLE IVS.tmpBus
ADD SOMECOLUMNAME varchar(500) Null
fetch next from cur_web into #webadressen
SET #counter = #counter + 1
END
Can anybody give me a syntax hint to this small problem?

Answer 1

You won't be able to parameterise the ALTER TABLE statement but you could build up the SQL and execute it something like this:
declare #sql nvarchar(max)
set #sql = 'create table IVS.tmpBus ( '
select
#sql = #sql + 'Webadresse_' +
row_number() over ( order by col ) +
' varchar(500) null, '
from sourceData
set #sql = substring(#sql, 1, len(#sql) - 2) + ' )'
exec #sql
Be careful about security/SQL-Injection attacks though.

Answer 2

Generally speaking DDL statements i.e. those that define tables and columns do not accept variables for table or column names.
You can sometimes get around that by preparing the statements but support for prepared DDL is not provided by all database engines.
The following example works in SQL Server 2005, although I would suggest that adding columns dynamically may not be the optimal solution
DECLARE #colname1 VARCHAR(10)
DECLARE #colname2 VARCHAR(10)
DECLARE #sql VARCHAR(MAX)
SET #colname1 = 'col1'
SET #colname2 = 'col2'
SET #sql = 'CREATE TABLE temptab (' + #colname1 + ' VARCHAR(10) )'
EXEC (#sql)
INSERT INTO temptab VALUES ('COl 1')
SET #sql = 'ALTER TABLE temptab ADD ' + #colname2 + ' VARCHAR(10)'
EXEC (#sql)
INSERT INTO temptab VALUES ('Col1', 'Col2')
SELECT * FROM temptab
DROP TABLE temptab
Produced the following results
col1 col2
---------- ----------
COl 1 NULL
Col1 Col2

Answer 3

i have Solved The Problem with the Non optimal Way.
The Code Works prefectly for me. i Hope another frustrated programmer can Use this.
DECLARE cur_web CURSOR FOR
SELECT IVS.LG_Webadressen.Adresse FROM IVS.LG_Webadressen WHERE IVS.LG_Webadressen.FK_GID = #welche
open cur_web /*Cursor wird geöffnet*/
fetch next from cur_web into #webadressen /*Erster Datensatz wird geholt*/
WHILE ##FETCH_STATUS = 0 /*Solange eine Datensatz vorhanden ist*/
BEGIN
/*Spalte Adden*/
SET #webadressenrow = 'Webadresse_'+CAST(#counter as nchar(1)) /*Anhängen des Durchlaufes an den Spaltennamen*/
SET #sql = 'ALTER TABLE IVS.temp_tbl ADD ' + #webadressenrow + ' VARCHAR(100)' /*Spalte adden*/
EXEC (#sql)
/*Wert für die Webadresse wird reingeschrieben*/
SET #sql = 'UPDATE IVS.temp_tbl Set ' + #webadressenrow + ' = ''' + #webadressen + ''' WHERE GID = ' + CAST(#welche as nchar(10)) + ''
EXEC(#sql)
/*nächtser Datensatz wird geholt*/
fetch next from cur_web into #webadressen
SET #counter = #counter + 1
END
/*Cursor zerstören und Schließen*/
CLOSE cur_web
DEALLOCATE cur_web