Delete tables script

Question

Delete tables script

a_ud

SSCrazy

Points: 2457
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February 23, 2011 at 2:55 am

#246105

Hi all,
I know this must be very basic, still a clear answer is appreciated.
I've been using a database made out of 2 separate SQL databases and more than 150 tables in total. Last task now is to delete most of the data (but not every table!) before going live.
Since I haven't used scripts before, my questions are:
1) How do you 'create' a delete a given table/s script? I mean the 1-2-3 steps process. Once done, how do you add new tables to that script, so that you can include as many as wanted?
2) Is it better to delete the data using Delete or Truncate?
3) If most of the tables are linked to an Access font-end, is there any technical advantage in deleting from SQL rather than from Access?
Thanks, a.

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spaghettidba SSC Guru Points: 105732 More actions · Answer 1

a_ud (2/23/2011)
1) How do you 'create' a delete a given table/s script? I mean the 1-2-3 steps process. Once done, how do you add new tables to that script, so that you can include as many as wanted?

You could use something like this (untried):

DECLARE @tabSchema sysname

DECLARE @tabName sysname

DECLARE @deleteSQL nvarchar(4000)

DECLARE @truncateSQL nvarchar(4000)

DECLARE tabs CURSOR STATIC LOCAL FORWARD_ONLY

FOR

SELECT TABLE_SCHEMA, TABLE_NAME

FROM INFORMATION_SCHEMA.TABLES

WHERE TABLE_TYPE = 'BASE TABLE'

AND TABLE_NAME IN (

'Table1',

'Table2',

...

'TableN'

)

OPEN tabs

FETCH NEXT FROM tabs INTO @tabSchema, @tabName

--UNCOMMENT IF YOU WANT THE TASK TO BE PERFORMED ATOMICALLY

--BEGIN TRANSACTION

WHILE @@FETCH_STATUS = 0

BEGIN

BEGIN TRY

--UNCOMMENT AND RUN THIS IF YOU HAVE A WHERE CONDITION

SET @deleteSQL = N'DELETE FROM ' + QUOTENAME(@tabSchema) + '.' + QUOTENAME(@tabName) + ' WHERE <condition>'

--EXEC(@deleteSQL)

--UNCOMMENT AND RUN THIS IF YOU WANT TO CLEAR THE WHOLE TABLE

SET @truncateSQL = N'TRUNCATE TABLE' + QUOTENAME(@tabSchema) + '.' + QUOTENAME(@tabName)

--EXEC(@truncateSQL)

END TRY

BEGIN CATCH

PRINT 'Unable to delete table ' + @tabName + ': ' + ERROR_MESSAGE()

--UNCOMMENT IF YOU WANT THE TASK TO BE PERFORMED ATOMICALLY

--ROLLBACK

END CATCH

FETCH NEXT FROM tabs INTO @tabSchema, @tabName

END

--UNCOMMENT IF YOU WANT THE TASK TO BE PERFORMED ATOMICALLY

--IF @@TRANCOUNT > 0 COMMIT

CLOSE tabs

DEALLOCATE tabs

2) Is it better to delete the data using Delete or Truncate?

Truncate is much faster and is the way to go if you want to clear the whole table.

3) If most of the tables are linked to an Access font-end, is there any technical advantage in deleting from SQL rather than from Access?

Deleting from Access could turn into a row-by-row deletion, I would avoid doing it.

Hope this helps

Gianluca

-- Gianluca Sartori

Lowell SSC Guru Points: 323518 More actions · Answer 2

this is a fairly common requirement.....delete lots of data, but not the look up tables kind of thing.

the things to watch out for are the order of the deletes...you can't delete parent data if you have child data referencing it due to the foreign key order.

you cannot issue the TRUNCATE command against a table that has foreign keys, so in that case you HAVE to delete.

here's a script i use to generate the commands...take a look at this, and see if this gives you a starting point for what you are asking.

SET NOCOUNT ON

--the list of tables we do not want to touch.

CREATE TABLE #Skipme(SchemaName varchar(255),TableName varchar(255))

INSERT INTO #Skipme

SELECT 'dbo','tbCity' UNION ALL

SELECT 'dbo','tbState' UNION ALL

SELECT 'dbo','tbCounty' UNION ALL

SELECT 'dbo','OtherLookupTables'

--adding some more based on a known factor: my lookup tables all start with LU* or TB*

INSERT INTO #Skipme

SELECT schema_name(schema_id),name from sys.objects

where LEFT(name,2) = 'LU'

OR LEFT(name,2) = 'TB'

--get the list of tables, and put them in FK hierarchy order.

CREATE TABLE #MyObjectHierarchy

(

HID int identity(1,1) not null primary key,

ObjectId int,

ObjectFullName varchar(510),

ObjectType int,

TypeDesc AS CASE

WHEN ObjectType = 1 THEN 'FUNCTION'

WHEN ObjectType = 4 THEN 'VIEW'

WHEN ObjectType = 8 THEN 'TABLE'

WHEN ObjectType = 16 THEN 'PROCEDURE'

WHEN ObjectType =128 THEN 'RULE'

ELSE ''

END,

ObjectSchema varchar(255),

ObjectName varchar(255),

DependencyOrder int

)

--our list of objects in dependancy order

INSERT #MyObjectHierarchy (ObjectType,ObjectName,ObjectSchema,DependencyOrder)

EXEC sp_msdependencies @intrans = 1

--Now simply select our results based on the joins of these two tables.

--the join excluses tables in out #SkipMe table

SELECT --*, --uncomment to see the full results.

CASE

WHEN DependencyOrder <= 1 --no dependancies at all

THEN 'TRUNCATE TABLE ' + QUOTENAME(T.ObjectSchema) + '.' + QUOTENAME(T.ObjectName)

ELSE 'DELETE FROM ' + QUOTENAME(T.ObjectSchema) + '.' + QUOTENAME(T.ObjectName)

END as cmd

FROM #MyObjectHierarchy T

LEFT OUTER JOIN #Skipme S

ON T.ObjectSchema = S.SchemaName

AND T.ObjectName = S.TableName

WHERE S.TableName IS NULL --not in the join

AND T.ObjectType = 8 --only tables

ORDER BY DependencyOrder,HID

--cleanup our temp tables

DROP TABLE #SkipMe

DROP TABLE #MyObjectHierarchy

Lowell

--help us help you! If you post a question, make sure you include a CREATE TABLE... statement and INSERT INTO... statement into that table to give the volunteers here representative data. with your description of the problem, we can provide a tested, verifiable solution to your question! asking the question the right way gets you a tested answer the fastest way possible!

a_ud SSCrazy Points: 2457 More actions · Answer 3

Thanks both, good replies that will be taken on board.