SQL - TRUNCATE TABLE Command

The SQL TRUNCATE TABLE command is used to delete all the records from an existing table by reinitiapzing the table s structure. This command instructs the database to deallocate the space for all records in a table and change this table s structure by resetting the table size. This is the reason why it is deemed to be a Data Definition Language (DDL) operation rather than Data Manipulation Language (DML), even though all the table data is removed.

Logically, the TRUNCATE TABLE statement performs similarly to the DELETE TABLE statement but without the WHERE clause. However, TRUNCATE is much faster than DELETE and does not allow roll back once committed.

You can also use DROP TABLE command to delete a table but it will remove the complete table structure from the database and you would need to re-create this table once again if you wish you store some data again.

Syntax

The basic syntax of a TRUNCATE TABLE command is as follows.

TRUNCATE TABLE table_name;

Example

In this example, let us first try to create a table with the name ‘CUSTOMERS’ which contains the personal details of customers including their name, age, address and salary etc. as shown below −

CREATE TABLE CUSTOMERS (
   ID INT NOT NULL,
   NAME VARCHAR (20) NOT NULL,
   AGE INT NOT NULL,
   ADDRESS CHAR (25),
   SALARY DECIMAL (18, 2),       
   PRIMARY KEY (ID)
);

Now insert values into this table using the INSERT statement as follows −

INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (1,  Ramesh , 32,  Ahmedabad , 2000.00 );

INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (2,  Khilan , 25,  Delhi , 1500.00 );

INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (3,  kaushik , 23,  Kota , 2000.00 );

INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (4,  Chaitap , 25,  Mumbai , 6500.00 );

INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (5,  Hardik , 27,  Bhopal , 8500.00 );

INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (6,  Komal , 22,  MP , 4500.00 );

INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY)
VALUES (7,  Muffy , 24,  Indore , 10000.00 );

The table will be created as follows −

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitap |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Muffy    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

Following query removes all the records of the customers table −

TRUNCATE TABLE CUSTOMERS;

Verification

Now, the CUSTOMERS table is truncated and the output from SELECT statement will be as shown in the code block below −

SELECT * FROM CUSTOMERS;

Empty set (0.00 sec)

TRUNCATE vs DELETE

Even though the TRUNCATE and DELETE commands work similar logically, there are some major differences that exist between them. They are detailed in the table below.

DELETE	TRUNCATE
The DELETE command in SQL removes one or more rows from a table based on the conditions specified in a WHERE Clause.	SQL s TRUNCATE command is used to remove all of the rows from a table, regardless of whether or not any conditions are met.
It is a DML(Data Manipulation Language) command.	It is a DDL(Data Definition Language) command.
There is a need to make a manual COMMIT after making changes to the DELETE command, for the modifications to be committed.	When you use the TRUNCATE command, the modifications made to the table are committed automatically.
It deletes rows one at a time and apppes some criteria to each deletion.	It removes all of the information in one go.
The WHERE clause serves as the condition in this case.	There is no necessity of using a WHERE Clause.
All rows are locked after deletion.	TRUNCATE utipzes a table lock, which locks the pages so they cannot be deleted.
It makes a record of each and every transaction in the log file.	The only activity recorded is the deallocation of the pages on which the data is stored.
It consumes a greater amount of transaction space compared to TRUNCATE command.	It takes comparatively less amount of transaction space.
If there is an identity column, the table identity is not reset to the value it had when the table was created.	It returns the table identity to a value it was given as a seed.
It requires authorization to delete.	It requires table alter permission.
When it comes to large databases, it is much slower.	It is faster.

TRUNCATE vs DROP

Unpke TRUNCATE that resets the table structure, DROP command completely frees the tablespace from the memory. They are both Data Definition Language operations as they interact with the definitions of database objects; which allows the database to automatically commit once these commands are executed with no chance to roll back.

However, there are still some differences existing between these commands as well, which are summarized in the following table −