Wednesday, October 19, 2011

Difference between Truncate and Delete command in SQL - Interview Questions with Example

Delete and truncate command in SQL
Truncate and delete in SQL
are two commands which is used to remove or delete data from table. Though quite basic in nature both sql commands can create lot of trouble until you are familiar with details before using it. Difference between Truncate and delete are not just important to understand perspective but also a very popular SQL interview topic which in my opinion a definite worthy topic. What makes them tricky is amount of data. Since most of Electronic trading system stores large amount of transactional data and some even maintain historical data, good understanding of delete and truncate command is required to effectively work on those environment.I have still seen people firing delete command just to empty a table with millions of records which eventually lock the whole table for doing anything and take ages to complete or Simply blew log segment or hang the machine.


Difference between truncate and delete command in sql
Most of enterprise stock trading system maintains two kind of database one transactional and other static. Transactional data is for day by day records which need to be purge at end of data or moved to historical data so that application can make a fresh start another day. If you need to work on such large set of data, my advice is to get clear and complete knowledge of delete and truncate command, along with there differences and when to use which command to remove data or purge tables.

In this article we will see where to use truncate in SQL and where to use delete in SQL, How to use truncate or delete and what danger or harm they can create if not used carefully along with difference between truncate and delete in SQL.

What is Truncate command in SQL

Use truncate table if you need to delete all rows, since truncate doesn't allow you to specify WHERE clause. truncate removes data by deallocating space used by table which removes lot of overhead in terms of logging and locking and that's why truncate is faster than delete.What you need to take care is rollback, data deleted by truncate can not be rolled back until data server specifically supports it e.g. MSSQL Server which allows to commit or rollback truncate table statement transactional. Another caveat with truncate table statement is that it doesn't fire a trigger and you can not truncate a table when a foreign key references any column to the table to be truncated. Only situation I see which is perfect for using truncate is purging tables with huge data, though there is another solution exists to drop table and recreated it if that make sense.

Example of truncate command in SQL

truncate table Orders;  //Order table shouldn't have a column which is foreign key on other table

What is Delete command in SQL

Delete is another sql command available for removing records from table. Delete is even more flexible than truncate like it provides support to WHERE Clause which can be use to remove selective data. It logs each row which allows operation to be rolled back and it also fires triggers. One disadvantage of using delete is speed and locking. Delete acquires lock on table and its also very slow operation because of logging, which makes it unsuitable for removing records from large tables. One workaround for this is batch-delete in which you remove batch of records instead on one record at a time. Delete is most suitable fore removing selective data and use it where you want to rollback transaction in database. It’s not useful to purge large amount of data from tables and should not be used, otherwise it could lock the table for very long time, blew log segment and can take ages to complete.

Example of delete command in SQL

delete  * from Orders; //delete all row from Orders, should not be used if Orders is large
delete  * from Orders where Symbol="MSFT.NQ" //delete all orders where symbol is MSFT.NQ


Difference between truncate and delete command in SQL

This is an important point to understand before using truncate or delete on production environment, or writing any script which purges data from tables.

1. truncate is fast delete is slow.
2. truncate doesn't do logging delete logs on per row basis.
3. rollback is possible with delete not with truncate until specifically supported by vendor.
4. truncate doesn't fire trigger, delete does.
5. Don't delete, truncate it when it comes to purge tables.
6. truncate reset identity column in table if any, delete doesn't.
7. truncate is DDL while delete is DML (use this when you are writing exam)
8. truncate doesn't support where clause, delete does.

So finally if you have table with huge data and want to empty it don’t Delete, truncate it


Interview questions on truncate and delete in SQL

Truncate and delete both are popular interview topics and there is always some question on these commands in SQL interview. Here I am listing some of SQL interview questions based on delete and truncate command in SQL, you can find answer in this article itself or by google.

1) If you have table which contains large amount of data which command will you use for removing data, truncate or delete?

2) What are differences between truncate and delete?

3) Which one is fast truncate or delete?

4) What is disadvantage of using truncate in sql?

5) How will you delete data if truncate is not supported and log segment is also not big enough to support complete delete?

6) Is there any way to remove data other than truncate and delete in SQL?

12 comments :

ARVIND said...

thanks a lot for the information really appreciate you,for spending your time in giving this useful information for many new learners like me.who are new to this field of study.

Javin @ unix command interview questions said...

@ARVIND, thanks for your comment. glad to hear that you like this truncate and delete tutorial on SQL.

SQL Beginner said...

difference between truncate and delete is one of the most popular SQL interview question and has been asked many times to many people but more than that if you know real differences between delete and truncate you can avoid mistakes while dealing with large and sensitive data. mostly in report writing, database cleanup, correct use of truncate and delete is must.

Ajith said...

GreT post. Seeing the link I was thinking whtz the big deal by it as I already know it but posts always tend to go that extra mile to make it more detailed . Thanks for tht . Also In case you are looking for new topics to blog,request to take a look at the google java interview questions . Questions like how to find matching strings from 2 string arrays If each array has 1 million records etc

Javin @ select query to get count said...

One of the important thing to note while using truncate is that it will reset your Identity column if you have any.

Vijay R said...

Hi Javin,

When you say truncate would reset the identity column, do you mean the foreign key which was referring to the table that was truncated? Could you explain this with examples.

Anonymous said...

truncate reset identity column means, if you last id was 100, after truncate it will start again with 1.

dev sangwan said...

database is large and truncate not supported..
log is not enough.. provide solution for this question plz.. m following this blog since last 1 month by recommendation of my teacher..n really i found much much beneficial and very clearly blog..thnks a lot..

Anonymous said...

using batch size we can delete rows from the table

Anonymous said...

wrt the below asked question
5) How will you delete data if truncate is not supported and log segment is also not big enough to support complete delete?

would Drop and create be a better option ??????

Javin @ Refrential integrity in database said...

@Anonymous, Yes, DROP is an alternative, if fits the criterion e.g. resetting of ID columns. When we DROP and recreate a table, ID column will restart from it's first value.

Shailendrasinh Chauhan said...

Nice information and very clearly provided.v good ####3

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