Oracle Table Functions

A table function is a PL/SQL function which will behave like a row source when queried. So you can perform transformations to the data before it is returned in the result set.

To write a table function, first we need to define an object as the row structure:

Because our function will return a table, we’ll also define a type:

How to Move a Datafile to Different Location

If the database is in archivelog mode, you can move the datafile while database is up. Take the datafile offline, copy (or move) the datafile to the new location:

Then issue the following commands to rename the datafile and make it online:

If the database is in no archivelog mode, you need to shutdown the database, open the database in mount mode and copy/move the datafile:

Then rename the file and open the database:

How to Prevent a User to Drop Own Objects

An Oracle user is always granted to drop their own objects. To prevent a user to drop their own objects, we can use DDL triggers.

Here’s sample trigger which will prevent HR user to drop EMPLOYEES table:

When HR user tries to drop employees table, he’ll see an ORA-20000 error.

For more information and samples about DDL triggers: http://psoug.org/reference/ddl_trigger.html

How to Limit the Number of Rows Returned in Oracle

I worked with MySQL for a long time, so I know how useful to limit the number of returning rows of a query. For example,

Unfortunately, Oracle does not have an operator like LIMIT. On the other hand, we can emulate it by using ROWNUM pseudo column. ROWNUM will tell the row number.

So we can limit the number of returning rows:

Unfortunately, if we add ORDER BY, GROUP BY operators to the query, this method will not work, because rownum will show the row number before “sorting”. So we use rownum in a subquery: