DELETE
The DELETE command is used to remove rows from a table. A WHERE clause can be used to only remove some rows. If no WHERE condition is specified, all rows will be removed. After performing a DELETE operation you need to COMMIT or ROLLBACK the transaction to make the change permanent or to undo it. Note that this operation will cause all DELETE triggers on the table to fire.
——————————————————————————————————–
COUNT(*)
———-
14
SQL> DELETE FROM emp WHERE job = ‘CLERK’;
4 rows deleted.
SQL> COMMIT;
Commit complete.
SQL> SELECT COUNT(*) FROM emp;
COUNT(*)
———-
10
——————————————————————————————————–
TRUNCATE
TRUNCATE removes all rows from a table. The operation cannot be rolled back and no triggers will be fired. As such, TRUCATE is faster and doesn’t use as much undo space as a DELETE.
——————————————————————————————————–
SQL> TRUNCATE TABLE emp;
Table truncated.
SQL> SELECT COUNT(*) FROM emp;
COUNT(*)
———-
0
——————————————————————————————————–
DROP
The DROP command removes a table from the database. All the tables’ rows, indexes and privileges will also be removed. No DML triggers will be fired. The operation cannot be rolled back.
——————————————————————————————————–
SQL> DROP TABLE emp;
Table dropped.
SQL> SELECT * FROM emp;
SELECT * FROM emp
ERROR at line 1:
ORA-00942: table or view does not exist
——————————————————————————————————–
Note : DROP and TRUNCATE are DDL commands, whereas DELETE is a DML command. As such, DELETE operations can be rolled back (undone), while DROP and TRUNCATE operations cannot be rolled back.
Difference between truncate and delete
| S.No. | Truncate | Delete |
|
1.
|
Truncate is a Data Definition Language | Delete is Data Manipulation Language |
|
2.
|
WHERE clause cannot be used with Truncate | WHERE clause can be used with Delete |
|
3.
|
It removes all the data | It deletes specified data if where condition exists |
|
4.
|
TRUNCATE TABLE always locks the table and page but not each row | DELETE statement is executed using a row lock, each row in table is locked for deletion. |
|
5.
|
Rollback is not possible | Rollback is possible |
|
6.**
|
Faster in performance wise, as it doesn’t keep any logs | Slower than truncate,as it keeps logs |
|
7.
|
TRUNCATE TABLE cannot activate triggers because the operation doesnot log individual row deletions | DELETE activates trigger because the operation logged individually |
|
8.
|
TRUNCATE don’t use check constraints | DELETE use check constraints ,if one exists then it shows error |
|
9.
|
It releases the storage space | It doesn’t release the storage space. |
**When you type DELETE.all the data get copied into the Rollback Tablespace first.then delete operation get performed.Thatswhy when you type ROLLBACK after deleting a table ,you can get back the data(The system get it for you from the Rollback Tablespace).All this process take time.But when you type TRUNCATE,it removes data directly without copying it into the Rollback Tablespace.Thatswhy TRUNCATE is faster.Once you Truncate you cann’t get back the data.