SQL Truncate vs. Delete

By now as you know, in this post I am going to discuss about two SQL commands we use regularly to clear data in our tables.

Usage

1. DELETE FROM TABLENAME
3. TRUNCATE TABLE TABLENAME

Even though they do a similar thing, there are some differences which makes them unique commands.

	DELETE	TRUNCATE
1.	Considered as Data Mining Language (DML) statement.	Considered as Data Definition Language (DDL) statement.
2.	Can be used to delete all or part of data in a table.	Will delete all the data in a table.
3.	Will log the actions.	There will be no logging on record deletion.
4.	Will use more locks.	Will use fewer locks.
5.	Will use more resources.	Will use less resources.
6.	Slow.	Fast.
7.	Will not reset the identity seed.	Will reset the identity seed to 0.
8.	Can be used to delete data in tables having relationships.	Cannot use in tables having relationships.
9.	Can be used in tables involved in log shipping or replication.	Cannot be used in tables involved in log shipping or replication.
10.	Transaction can be rolled back.	No rollback.
11.	Table may keep the empty pages. Can be released by running, SHRINKDATABASE (Database Name).	Data pages related to the table will be de allocated and returned to the system.
12.	Related Triggers are fired.	Triggers are not fired.

 

If you used DELETE to fully remove all the records, you can reset the identity value by running the following command.

1. DBCC CHECKIDENT (TABLENAME, RESEED, 0)

If you do have some rows left in table, simply replace 0 with the last identity column value. For example if you put 5 instead of 0 then the next record inserted into the table will have an identity column value of 6.

If you jus need to check the current identity value just use the following command.

1. DBCC CHECKIDENT (TABLENAME, NORESEED)

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Importing Data into SQL

If you need to import data from a file, this can be achieved by using SQL Bulk Insert command. Recently I did use this method to import some 500 000 data from few comma separated value (CSV) files and thought to share it with you. What you need to remember is, if you are importing data from more than one file the data should have the same format through out the files.

This is a sample set of data I imported into my table from the file named File1.csv.

Login,Name,Date,Result,Pass

U0001,Roman Silva,1/10/2010 17:23,100,TRUE

U0002,Anthony Don,28/09/2010 10:01,70,TRUE

U0003,Saman Perera,16/09/2010 11:31,90,TRUE

U0004,Silvia Raz,26/09/2010 22:11,40,FALSE

U0005,Rebecca Maine,18/09/2010 11:30,100,TRUE

I used the following script to create a temporary table for my imported data.

1. SET ANSI_NULLS ON
2. GO
3.  
4. SET QUOTED_IDENTIFIER ON
5. GO
6.  
7. CREATE TABLE [dbo].[TABLENAME_ImportedData](
8.     [LoginId] [NVARCHAR](50) NOT NULL,
9.     [Name] [NVARCHAR](200) NULL,
10.     [Date] [DATETIME] NULL,
11.     [Result] [INT] NULL,
12.     [Pass] [NVARCHAR](8) NULL
13. ) ON [PRIMARY]
14.  
15. GO

To fetch data from the file I used the following script.

1. BULK INSERT [TABLENAME_ImportedData]
2. FROM 'D:\DataFiles\File1.csv'
3. WITH (
4.          FIELDTERMINATOR =',', — Since my columns are seperated using commas (,).
5.          ROWTERMINATOR ='\n',  — Since each data row is in its own line.
6.          FIRSTROW = 2          — Since my first row is having column names. Please note FIRSTROW is not recommended to skip the column names.
7.       )

While running the script I faced an issue with the date since my server was set to US English as the default language. In US English the dates should be in MDY format. So I had 3 choices, either to change the date formats on my data files, change the server default language to another language which has its date format as DMY or change the date format of the server. I used the easy way to change the date format of the server by running the following command.

1. SET DATEFORMAT dmy

If you like to change SQL server default language and need help please read my article on that.

Changing SQL Server Default Language

If you needed to change the default language of a SQL Server login you can do so by using sp_defaultlanguage command.

First determine the current default language by using the @@language variable as follows.

1. SELECT @@language

Then choose the language you want to change to from the available languages. You can find the available languages by the following command.

1. EXEC sys.sp_helplanguage

After selecting the language wanted to change, run the following command to change the default language, I am changing it to British using the following command.

1. DECLARE @Login NVARCHAR(30)
2. SET @Login = SYSTEM_USER
3. EXEC sp_defaultlanguage @Login, 'British'

Run the @@language to see whether the default language is changed as you wanted. Do not forget to use a new query window to see the changes.

UPDATE with JOIN

If you need to update data of a table (DestinationTable) with data of another table (SourceTable) there are 3 ways you can get this done in SQL.

Method 1

This is the most common and simple way with using a sub query.

1. UPDATE DestinationTable SET DestinationColumn =
2.     (SELECT ValueColumn FROM SourceTable WHERE DestinationTable.MappingColumn = SourceTable.MappingColumn)

 

Method 2

This users the most common FROM clause to join the two tables as shown below.

1. UPDATE DestinationTable SET DestinationColumn = SourceTable.ValueColumn
2. FROM SourceTable
3. WHERE DestinationTable.MappingColumn = SourceTable.MappingColumn

 

Method 3

Last method mentioned below uses the join clause to join both the tables to make the update happen properly.

1. UPDATE DestinationTable SET DestinationColumn = SourceTable.ValueColumn
2. FROM DestinationTable
3. INNER JOIN SourceTable ON DestinationTable.MappingColumn = SourceTable.MappingColumn

 

Hope these helps.

Database Not Accessible

Recently after restoring a database backup which I got from another server I repeatedly encountered an error when logged in using a user who is given permission. But when ‘sa’ account is used I could work with the database without any problem.

The error message was “The database DATABASE NAME is not accessible. (ObjectExplorer)” which was not helpful since it didn’t give any clue to figure out the issue.

Later I found that the issue is because the user in the restored database is not properly mapped to the user in the new server and the resolution for this is to run the stored procedure sp_change_users_login to correct the orphaned user.

• sp_change_users_login ‘update_one’, ‘USER’, ‘LOGIN’ – Links the given user in the current database to the specified login.
• sp_change_users_login ‘auto_fix’, ‘USER’ – Links the given user in the current database to the login having the same name in the current server.

After running this you will be able to access the restored database without any issue using the mentioned database user.