Truncate/Clear table variable in SQL Server 2008

Is it possible to truncate or flush out a table variable in SQL Server 2008?

declare @tableVariable table
(
id int,
value varchar(20)
)


while @start <= @stop
begin
insert into @tableVariable(id, value)
select id
, value
from xTable
where id = @start
    

--Use @tableVariable


--@tableVariable should be flushed out of
-- old values before inserting new  values


set @start = @start + 1
end
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最佳答案

just delete everything

DELETE FROM @tableVariable
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No, you cannot TRUNCATE a table variable since it is not a physical table. Deleting it would be faster. See this answer from Aaron Bertrand.

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Table variables do not support TRUNCATE syntax - the only way of truncating them is implicitly by letting them fall out of scope.

Both temporary tables and table variables can be cached when used in stored procedures and the below may well end up with the same table variable being used after truncation rather than an actual drop and create

CREATE PROC dbo.foo @start INT
AS
BEGIN
DECLARE @tableVariable TABLE (
id    INT,
value VARCHAR(20))


INSERT INTO @tableVariable
(id,
value)
SELECT id,
value
FROM   xTable
WHERE  id = @start;
--Use @tableVariable
END


GO


WHILE @start <= @stop
BEGIN
EXEC dbo.foo @start


SET @start = @start + 1
END

Of course a far easier alternative would be to switch to using a #temp table instead as that supports TRUNCATE directly.

DML on both table variables and temp tables writes to the tempdb transaction log. Whether or not it is worth switching to TRUNCATE rather than DELETE depends on the size of data involved. TRUNCATE will just log the page deallocations. DELETE will log the actual deleted values. One other difference between the two is that TRUNCATE deallocates the last page from the table and DELETE doesn't. If only a small quantity of data is inserted and deleted in each loop iteration then the overhead from logging the deleted rows can be less than the overhead from constantly deallocating and reallocating the single page in the table.

Conversely if you will be inserting and deleting large amounts of data on each iteration you may find that TRUNCATE not only makes the operation of deleting all rows more efficient but also can benefit the subsequent insert statement.

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I know this is an old question but i've figured a way to do this. we had tables with millions of rows and didn't want to delete them due to transaction log space.

Create a procedure that you pass in the table name you want to truncate, the procedure will create another procedure that does the trucate and then deletes the procedures.

USE [My_Database]
GO
/****** Object:  StoredProcedure [dbo].[ClearOutTable_p1]    Script Date: 23/09/2015 09:03:14 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO


-- =============================================
-- Author:      Oraclebhoy
-- Create date: 23/09/2015
-- Description:
--
-- removes the content of the table passed in through the parameter
-- =============================================
create procedure [dbo].[ClearOutTable_p1]
@tablename varchar(max)
as


-- CREATE THE TRUNCATE STATEMENT PASSING IN TABLE VARIABLE
declare @truncatesql varchar(max)
set @truncatesql = 'truncate table ' + @tablename


-- IF PROCEDURE EXISTS DROP
if exists (select name from sys.all_objects where name = 'ClearOutTable_TEMP'and type = 'P')
begin
drop procedure [dbo].[ClearOutTable_TEMP]
end


-- CREATE TEMP PROCEDURE
exec ('create procedure [dbo].[ClearOutTable_TEMP]
as
'+@truncatesql+'')


-- EXECUTE THE PROCEDURE
exec [dbo].[ClearOutTable_TEMP]


-- DROP THE PROCEDURE
drop procedure [dbo].[ClearOutTable_TEMP]

Hope this helps.

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I'd add to the "technically" correct answer of using DELETE @VariableTable that if you happen to also have an Identity-Field in your @Table Variable (e.g. i int (1,1)) and you'd like to re-use this table (even if you re-declare it in a loop) it is still within scope and there it no way to reseed it either.

See: Table Variable Identity Column

It is best to use #TempTable in these cases - then you may Truncate or use DBCC to reseed.
You will reap performance improvements with Truncate and be able to create additional indexes.
I think the rule of thumb is, if you're ever going to delete everything using DELETE @VariableTable, then you've introduced a code-smell that says, you should have used #TempTable and TRUNCATE instead.

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--Usage: exec sp_truncateifexists tablename
CREATE PROCEDURE sp_truncateifexists
@tableVariable nvarchar(200)
AS
BEGIN
IF EXISTS (
SELECT 1
FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_NAME = @tableVariable )
BEGIN
DECLARE @query nvarchar(250)
SET @query = 'TRUNCATE TABLE ' + @tableVariable
EXECUTE (@query)
END
END
GO

Remember to use #temp tables if you don't need the tables later.

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This is other limitation of the Table variable . Most of the cases we use Truncate command where we want Identity of the table needs to be reset. Suppose you are inserting values into Table variable inside cursor & you need to clear table variable , for each insertion new identity will be populated . In such scenarios , we need to use #temp to get new identity for each fetch for the cursor


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