While working with the database we often need to delete data, sometimes to create more space, sometimes just remove all data to make the table ready for the next day's transaction, or sometimes just selectively remove stale data. SQL provides a couple of handy commands to remove data e.g. truncate, delete and drop. The last one is a bit different because instead of removing data it just deletes the table. What is the difference between truncate and delete command in SQL or when to use truncate vs delete is one of the most frequently asked SQL interview questions?
Knowledge of this critical command is not only useful for interviews but also while working with a big database with a large number of records, especially while writing SQL scripts for purging transaction databases.
An Incorrect choice of SQL command can result in either very slow processor can even blow up the log segment; if too much data needs to be removed and log segment is not enough. That's why it's critical to know when to use the truncate and delete command in SQL.
In this article, we will first see few differences between them and based upon them, you should be able to find out when DELETE is a better option for removing data or TRUNCATE should be used to purge tables.
1. The first and most important difference between TRUNCATE and DELETE command in SQL is that truncate doesn't log row level details while delete is logged. Since TRUNCATE is not logged, it's not possible to rollback it e.g. in Oracle. But some database may provide rollback functionality for truncate, on the other hand, DELETE is always logged and can be rolled back.
2) Due to above reason, TRUNCATE command is much faster than DELETE, and due to same reason, you should not use DELETE to remove large data set, since every delete operation is logged, it may be possible that your log segment gets filled and blew up, if you try to empty a large table using DELETE command.
3) One syntactical difference between DELETE and TRUNCATE is that former is a DML command while later is a DDL command.
4) Another critical difference between TRUNCATE and DELETE command in SQL is that former reset any Identity column while DELETE retains the value of identity column.
5) DELETE command work at row level and acquire the row level lock while TRUNCATE locks the whole table, instead of individual rows.
6) One more difference between TRUNCATE vs DELETE comes from the fact we discussed in the first option, it doesn't activate the trigger, because it doesn't log individual row deletion, while DELETE activates the trigger, because of row level logging.
7) Truncate is usually used to remove all data from tables like in order to empty a particular table and you can define any filter based upon WHERE clause, but with DELETE, you can define the condition in WHERE clause for removing data from tables.
8) Since TRUNCATE is a DDL operation, it's automatically get committed, on the other hand, DELETE is not auto-commit.
In this article, we will first see few differences between them and based upon them, you should be able to find out when DELETE is a better option for removing data or TRUNCATE should be used to purge tables.
TRUNCATE vs DELETE in SQL
As I said both commands are used to remove data in SQL, DELETE is more common than TRUNCATE as it's part of the essential CRUD (Create, Read, Update, and Delete) operation, but there are subtle differences between them. Let's see them first :1. The first and most important difference between TRUNCATE and DELETE command in SQL is that truncate doesn't log row level details while delete is logged. Since TRUNCATE is not logged, it's not possible to rollback it e.g. in Oracle. But some database may provide rollback functionality for truncate, on the other hand, DELETE is always logged and can be rolled back.
2) Due to above reason, TRUNCATE command is much faster than DELETE, and due to same reason, you should not use DELETE to remove large data set, since every delete operation is logged, it may be possible that your log segment gets filled and blew up, if you try to empty a large table using DELETE command.
3) One syntactical difference between DELETE and TRUNCATE is that former is a DML command while later is a DDL command.
4) Another critical difference between TRUNCATE and DELETE command in SQL is that former reset any Identity column while DELETE retains the value of identity column.
5) DELETE command work at row level and acquire the row level lock while TRUNCATE locks the whole table, instead of individual rows.
6) One more difference between TRUNCATE vs DELETE comes from the fact we discussed in the first option, it doesn't activate the trigger, because it doesn't log individual row deletion, while DELETE activates the trigger, because of row level logging.
7) Truncate is usually used to remove all data from tables like in order to empty a particular table and you can define any filter based upon WHERE clause, but with DELETE, you can define the condition in WHERE clause for removing data from tables.
8) Since TRUNCATE is a DDL operation, it's automatically get committed, on the other hand, DELETE is not auto-commit.
Summary
Here is a nice table to remember the difference between truncate and delete command in SQL
That's all on the difference between TRUNCATE and DELETE command in SQL. As we learned, you should truncate if you want to completely empty your tables, if truncate is not what you want then, you should look at the batch delete option. Batch delete is actually a little faster than deleting individual rows and also doesn't blew log segment.
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