Difference between TRUNCATE, DELETE and DROP commands

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.

 DELETE FROM emp;

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.Truncate is also delete the records but it is not delete the table space which is created by the data base.

TRUNCATE TABLE emp;

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.We can not recover the table before Oracle 10g. But Oracle 10g provide the command to recover it by using the command (FLASHBACK)
Drop command will delete the entire row also the structure.But truncate will delete the contenets only not the strucure, so no need to give specifications for another table creation.

 DROP TABLE emp;

how to find 2nd highest salary

select Max(Salary) from tblSalaryDetails  where salary   Not IN(select Max(Salary) from tblSalaryDetails )


for 6 ,7 and other

select max(salary) from EmployeeDetails

where Salary NOT IN
(
SELECT TOP 5 (SALARY) from EmployeeDetails ORDER BY Salary Desc)

SQL - Difference between Truncate & Delete

1. TRUNCATE is a DDL (Data Definition Language) command and DELETE is a DML (Data Manipulation Language) command.

2. You can use WHERE clause with DELETE but not with TRUNCATE .

3. You cannot rollback data in TRUNCATE but in DELETE it is possible to rollback data.

4. A trigger doesn't get fired in case of TRUNCATE whereas Triggers get fired in case of a DELETE command.

5. TRUNCATE is faster than DELETE. TRUNCATE is faster than DELETE due to the way TRUNCATE "removes" rows. Actually, TRUNCATE does not remove data, but rather deallocates whole data pages and removes pointers to indexes. The data still exists until it is overwritten or the database is shrunk. This action does not require a lot of resources and is therefore very fast.

6. TRUNCATE resets the Identity counter if there is any identity column present in the table where DELETE does not reset the identity counter.

7. You cannot TRUNCATE a table that has any foreign key constraints. You will have to remove the contraints, TRUNCATE the table, and reapply the contraints.

8. DELETE and TRUNCATE operations are both logged. DELETE is a logged operation on a per row basis and TRUNCATE command logs the deallocation of the data