Db2 TRUNCATE TABLE

Summary: in this tutorial, you will learn how to use the Db2 TRUNCATE TABLE statement to delete all rows from a table.

Introduction to Db2 TRUNCATE TABLE statement

The DELETE statement without a WHERE clause allows you to delete all rows from a table. However, if you have a table with a large volume of data, the DELETE statement will not be efficient.

Fortunately, Db2 provides the TRUNCATE TABLE that also delete all rows from a table, but in a more efficient way.

The following shows the syntax of the TRUNCATE TABLE statement:

TRUNCATE TABLE table_name
[DROP STORAGE | REUSE STORAGE]
[IGNORE DELETE TRIGGERS | RESTRICT WHEN DELETE TRIGGERS]
[IMMEDIATE]

In this syntax:

First, specify the name of the table from which you want to delete all data.

Second, the DROP STORAGE or REUSE STORAGE option determines whether Db2 should drop or reuse the existing storage allocated for the table.

• The DROP STORAGE releases all storages allocated for the table and make the storage available for use for the same table or any other tables within the tablespace.
• The REUSE STORAGE makes all storages empty, but continues allocating for the table.

The default option is DROP STORAGE.

Third, the IGNORE DELETE TRIGGERS and RESTRICT WHEN DELETE TRIGGERS options determine the behavior of the truncate operation when the table has associated delete triggers.

• The IGNORE DELETE TRIGGERS option will not fire any trigger defined on the table.
• The RESTRICT WHEN DELETE TRIGGERS option will result in an error if the table has one or more deleted triggers.

By default, the TRUNCATE TABLE statement uses IGNORE DELETE TRIGGERS option.

Fourth, the IMMEDIATE option, if specified, will delete all rows from the table without the ability to roll back. On the other hand, if you don’t specify the IMMEDIATE option, you can use a ROLLBACK statement to undo the truncate operation.

Notice that when you use the TRUNCATE TABLE statement in a transaction, you must place it as the first statement.

Db2 TRUNCATE TABLE examples

Let’s take some examples of using the TRUNCATE TABLE statement.

First, create a new table named books2 that has the same structure as the books table:

CREATE TABLE books2 LIKE books;

Then, insert all rows from the books table to the books2 table using the INSERT INTO SELECT statement:

INSERT INTO books2 
SELECT * FROM books;

1) Using Db2 TRUNCATE TABLE statement within a transaction example

First, start a new transaction:

BEGIN TRANSACTION;

Second, truncate the book2 table:

TRUNCATE TABLE books2;

Third, rollback the transaction:

ROLLBACK;

Fourth, verify the truncate operation by querying some rows from the books2 table:

SELECT
	title
FROM
	books2
ORDER BY 
	title
LIMIT 5;

Here is the output:

As you can see clearly from the output, the truncate operation on the books2 table was rolled back successfully.

2) Using Db2 TRUNCATE TABLE statement to truncate a table immediately

The following example uses the TRUNCATE TABLE statement to truncate the books2 table immediately with the IMMEDIATE option:

TRUNCATE TABLE books2 
IMMEDIATE;

If you query data from the books2 table, you will get an empty result set:

SELECT * FROM books2;

In this tutorial, you’ve learned how to use the Db2 statement to delete all data from a table.