Please see the code below:
select top 1 * into #dbusers from dbusers
declare #tsql as varchar(1000)
set #tsql = 'select * from #dbusers'
exec (#tsql)
This works as I would expect i.e. one row is returned by the dynamic SQL. Is it possible to do this:
declare #tsql as varchar(1000)
set #tsql = 'select top 1 * into #dbusers from dbusers'
exec (#tsql)
select * from #dbusers
Here I get the error:
Invalid object name '#dbusers'
Is there a workaround?
I realise that you can have output parameters with dynamic SQL. However, I also know that when using stored procedures you cannot return tables as output parameters.
Is it possible to do this? Is there a workaround (except creating a physical table)?
Temporary tables are only available within the session that created them. With Dynamic SQL this means it is not available after the Dynamic SQL has run. Your options here are to:
Create a global temporary table, that will persist outside your session until it is explicitly dropped or cleared out of TempDB another way, using a double hash: create table ##GlobalTemp
Because this table persists outside your session, you need to make sure you don't create two of them or have two different processes trying to process data within it. You need to have a way of uniquely identifying the global temp table you want to be dealing with.
You can create a regular table and remember to drop it again afterwards.
Include whatever logic that needs to reference the temp table within the Dynamic SQL script
For your particular instance though, you are best off simply executing a select into which will generate your table structure from the data that is selected.
I have a problem with generating a particular table on the fly due to expensive SQL requests. I would like to pre-generate the table and simply display it to the user. The problem is: there are multiple versions of the table, and new ones will be continuously added.
Please give me some ideas on how to design a table (?) to hold these tables.
One idea that I have is to append a version number to each row in the individual tables and dump them all into a single cache table. This way, I can easily display just the requested version by filtering on version number. Is there a better way?
Without an example I may be making some assumptions here, but it sounds to me like dynamic SQL would do the trick.
declare #sql varchar(max)
select
#sql = 'SELECT * FROM MyTableName_' + v.version
from dbo.Version as v
where v.id = 1
exec (#sql)
If you end up using it, just know that dynamic sql is a cruel mistress. With one hand she give'th, the other she take'th away.
I have about half a dozen generic, but fairly complex stored procedures and functions that I would like to use in a more generic fashion.
Ideally I'd like to be able to pass the table name as a parameter to the procedure, as currently it is hard coded.
The research I have done suggests I need to convert all existing SQL within my procedures to use dynamic SQL in order to splice in the dynamic table name from the parameter, however I was wondering if there is a easier way by referencing the table in another way?
For example:
SELECT * FROM #MyTable WHERE...
If so, how do I set the #MyTable variable from the table name?
I am using SQL Server 2005.
Dynamic SQL is the only way to do this, but I'd reconsider the architecture of your application if it requires this. SQL isn't very good at "generalized" code. It works best when it's designed and coded to do individual tasks.
Selecting from TableA is not the same as selecting from TableB, even if the select statements look the same. There may be different indexes, different table sizes, data distribution, etc.
You could generate your individual stored procedures, which is a common approach. Have a code generator that creates the various select stored procedures for the tables that you need. Each table would have its own SP(s), which you could then link into your application.
I've written these kinds of generators in T-SQL, but you could easily do it with most programming languages. It's pretty basic stuff.
Just to add one more thing since Scott E brought up ORMs... you should also be able to use these stored procedures with most sophisticated ORMs.
You'd have to use dynamic sql. But don't do that! You're better off using an ORM.
EXEC(N'SELECT * from ' + #MyTable + N' WHERE ... ')
You can use dynamic Sql, but check that the object exists first unless you can 100% trust the source of that parameter. It's likely that there will be a performance hit as SQL server won't be able to re-use the same execution plan for different parameters.
IF OBJECT_ID(#tablename, N'U') IS NOT NULL
BEGIN
--dynamic sql
END
ALTER procedure [dbo].[test](#table_name varchar(max))
AS
BEGIN
declare #tablename varchar(max)=#table_name;
declare #statement varchar(max);
set #statement = 'Select * from ' + #tablename;
execute (#statement);
END
It's not hard to find developers who think cursors are gauche but I am wondering how to solve the following problem without one:
Let's say I have a proc called uspStudentDelete that takes as a parameter #StudentID.
uspStudentDelete applies a bunch of cascading soft delete logic, marking a flag on tables like "classes", "grades", and so on as inactive. uspStudentDelete is well vetted and has worked for some time.
What would be the best way to run uspStudentDelete on the results of a query (e.g. select studentid from students where ... ) in TSQL?
That's exactly what cursors are intended for:
declare c cursor local for <your query here>
declare #ID int
open c
fetch next from c into #id
while ##fetch_status = 0
begin
exec uspStudentDelete #id
fetch next from c into #id
end
close c
deallocate c
Most people who rail against cursors think you should do this in a proper client, like a C# desktop application.
The best solution is to write a set-based proc to handle the delete (try running this through a cursor to delete 10,000 records and you'll see why) or to add the set-based code to the current proc with a parameter to tell you wheter to run the set-based or single record part of the proc (this at least keeps it together for maintenance purposes).
In SQL Server 2008 you can use a table variable as an input variable. If you rewrite the proc to be set-based, you can have the same logic and run it no matter if the proc sends in one record or ten thousand. You may need to have a batch process in there to avoid deleting millions of records in one go though and locking up the tables for hours. Of course if you do this you will also need to adjust how the currect sp is being called.
I just ran into a strange thing...there is some code on our site that is taking a giant SQL statement, modifying it in code by doing some search and replace based on some user values, and then passing it on to SQL Server as a query.
I was thinking that this would be cleaner as a parameterized query to a stored proc, with the user values as the parameters, but when I looked more closely I see why they might be doing it...the table that they are selecting from is variably dependant on those user values.
For instance, in one case if the values were ("FOO", "BAR") the query would end up being something like "SELECT * FROM FOO_BAR"
Is there an easy and clear way to do this? Everything I'm trying seems inelegant.
EDIT: I could, of course, dynamically generate the sql in the stored proc, and exec that (bleh), but at that point I'm wondering if I've gained anything.
EDIT2: Refactoring the table names in some intelligent way, say having them all in one table with the different names as a new column would be a nice way to solve all of this, which several people have pointed out directly, or alluded to. Sadly, it is not an option in this case.
First of all, you should NEVER do SQL command compositions on a client app like this, that's what SQL Injection is. (Its OK for an admin tool that has no privs of its own, but not for a shared use application).
Secondly, yes, a parametrized call to a Stored procedure is both cleaner and safer.
However, as you will need to use Dynamic SQL to do this, you still do not want to include the passed string in the text of the executed query. Instead, you want to used the passed string to look up the names of the actual tables that the user should be allowed to query in the way.
Here's a simple naive example:
CREATE PROC spCountAnyTableRows( #PassedTableName as NVarchar(255) ) AS
-- Counts the number of rows from any non-system Table, *SAFELY*
BEGIN
DECLARE #ActualTableName AS NVarchar(255)
SELECT #ActualTableName = QUOTENAME( TABLE_NAME )
FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_NAME = #PassedTableName
DECLARE #sql AS NVARCHAR(MAX)
SELECT #sql = 'SELECT COUNT(*) FROM ' + #ActualTableName + ';'
EXEC(#SQL)
END
Some have fairly asked why this is safer. Hopefully, little Bobby Tables can make this clearer:
0
Answers to more questions:
QUOTENAME alone is not guaranteed to be safe. MS encourages us to use it, but they have not given a guarantee that it cannot be out-foxed by hackers. FYI, real Security is all about the guarantees. The table lookup with QUOTENAME, is another story, it's unbreakable.
QUOTENAME is not strictly necessary for this example, the Lookup translation on INFORMATION_SCHEMA alone is normally sufficient. QUOTENAME is in here because it is good form in security to include a complete and correct solution. QUOTENAME in here is actually protecting against a distinct, but similar potential problem know as latent injection.
I should note that you can do the same thing with dynamic Column Names and the INFORMATION_SCHEMA.COLUMNS table.
You can also bypass the need for stored procedures by using a parameterized SQL query instead (see here: https://learn.microsoft.com/en-us/dotnet/api/system.data.sqlclient.sqlcommand.parameters?view=netframework-4.8). But I think that stored procedures provide a more manageable and less error-prone security facility for cases like this.
(Un)fortunately there's no way of doing this - you can't use table name passed as a parameter to stored code other than for dynamic sql generation. When it comes to deciding where to generate sql code, I prefer application code rather that stored code. Application code is usually faster and easier to maintain.
In case you don't like the solution you're working with, I'd suggest a deeper redesign (i.e. change the schema/application logic so you no longer have to pass table name as a parameter anywhere).
I would argue against dynamically generating the SQL in the stored proc; that'll get you into trouble and could cause injection vulnerability.
Instead, I would analyze all of the tables that could be affected by the query and create some sort of enumeration that would determine which table to use for the query.
Sounds like you'd be better off with an ORM solution.
I cringe when I see dynamic sql in a stored procedure.
One thing you can consider is to make a case statement that contains the same SQL command you want, once for each valid table, then pass as a string the table name into this procedure and have the case choose which command to run.
By the way as a security person the suggestion above telling you to select from the system tables in order to make sure you have a valid table seems like a wasted operation to me. If someone can inject passed the QUOTENAME() then then injection would work on the system table just as well as on the underlying table. The only thing this helps with it to ensure it is a valid table name, and I think the suggestion above is a better approach to that since you are not using QUOTENAME() at all.
Depending on whether the set of columns in those tables is the same or different, I'd approach it in two ways in the longer term:
1) if they the same, why not create a new column that would be used as a selector, whose value is derived from the user-supplied parameters ? (is it a performance optimization?)
2) if they are different, chances are that handling of them is also different. As such, it seems like splitting the select/handle code into separate blocks and then calling them separately would be a most modular approach to me. You will repeat the "select * from" part,
but in this scenario the set of tables is hopefully finite.
Allowing the calling code to supply two arbitrary parts of the table name to do a select from feels very dangerous.
I don't know the reason why you have the data spread over several tables, but it sounds like you are breaking one of the fundamentals. The data should be in the tables, not as table names.
If the tables have more or less the same layout, consider if it would be best to put the data in a single table instead. That would solve your problem with the dynamic query, and it would make the database layout more flexible.
Instead of Querying the tables based on user input values, you can pick the procedure instead.
that is to say
1. Create a procedure FOO_BAR_prc and inside that you put the query 'select * from foo_bar' , that way the query will be precompiled by the database.
2. Then based on the user input now execute the correct procedure from your application code.
Since you have around 50 tables, this might not be a feasible solution though as it would require lot of work on your part.
In fact, I wanted to know how to pass table name to create a table in stored procedure. By reading some of the answers and attempting some modification at my end, I finally able to create a table with name passed as parameter. Here is the stored procedure for others to check any error in it.
USE [Database Name]
GO
/****** Object: StoredProcedure [dbo].[sp_CreateDynamicTable] Script Date: 06/20/2015 16:56:25 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE PROCEDURE [dbo].[sp_CreateDynamicTable]
#tName varchar(255)
AS
BEGIN
SET NOCOUNT ON;
DECLARE #SQL nvarchar(max)
SET #SQL = N'CREATE TABLE [DBO].['+ #tName + '] (DocID nvarchar(10) null);'
EXECUTE sp_executesql #SQL
END
#RBarry Young
You don't need to add the brackets to #ActualTableName in the query string because it is already included in the result from the query in the INFORMATION_SCHEMA.TABLES. Otherwise, there will be error(s) when executed.
CREATE PROC spCountAnyTableRows( #PassedTableName as NVarchar(255) ) AS
-- Counts the number of rows from any non-system Table, SAFELY
BEGIN
DECLARE #ActualTableName AS NVarchar(255)
SELECT #ActualTableName = QUOTENAME( TABLE_NAME )
FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_NAME = #PassedTableName
DECLARE #sql AS NVARCHAR(MAX)
--SELECT #sql = 'SELECT COUNT(*) FROM [' + #ActualTableName + '];'
-- changed to this
SELECT #sql = 'SELECT COUNT(*) FROM ' + #ActualTableName + ';'
EXEC(#SQL)
END
I would avoid dynamic SQL at all costs.
Isn't the most elegant solution but does the job perfectly.
PROCEDURE TABLE_AS_PARAMTER (
p_table_name IN VARCHAR2
) AS
BEGIN
CASE p_table_name
WHEN 'TABLE1' THEN
UPDATE TABLE1
SET
COLUMN1 =1
WHERE
ID =1;
WHEN 'TABLE2' THEN
UPDATE TABLE1
SET
COLUMN1 =1
WHERE
ID =2;
END CASE;
COMMIT;
EXCEPTION
WHEN OTHERS THEN
ROLLBACK
END TABLE_AS_PARAMTER;