Oracle EMPTY_CLOB Function with Examples

In this article, I’ll explain what the EMPTY_CLOB function does and show you an example of how to use it. Purpose of the Oracle EMPTY_CLOB Function The EMPTY_CLOB function is used to initalise a CLOB column to EMPTY. It can be used in several places: In an INSERT statement In an UPDATE statement Initialising a […]

How can we use Oracle to deduplicate our data

Dear Patrick,

We have gone through a merger at our company where we are trying to merge the databases. The problem now is that we have duplicate records in our tables. We can of course go through all the records by hand and check if they exist twice. Another option is to build an application to do this. But using the Oracle Database there must be a better way to do this. Any ideas?

Ramesh (more...)

OTN Appreciation Day: Analytic Functions

This is my contribution to the OTN Appreciation Day, which was initiated by Tim Hall.

One of my favorite features of the Oracle Database are Analytic Functions. They were introduced with Oracle Database 8.1.6 Enterprise Edition, and have been in the Standard Edition since version 9.

With analytic functions you can add inter-row calculations, aggegrate over multiple dimensions, rank assignments based on values. All this without a GROUP BY clause.
The syntax might (more...)

Oracle EMPTY_BLOB Function with Examples

In this article, I’ll explain what the Oracle EMPTY_BLOB function does and show you an example of how to use it. Purpose of the Oracle EMPTY_BLOB Function The EMPTY_BLOB function is used to initalise a LOB column to EMPTY. It can be used in several places: In an INSERT statement In an UPDATE statement Initialising […]

Restriction when column default is sequence.nextval

Oracle 12c introduced the ability to specify sequence.nextval as the default on a column, which is really nice – including the fact that it eliminates one of your excuses why you don’t decommission those old triggers.

Unfortunately it doesn’t work as you might expect if you use an INSERT ALL statement; it evaluates the default expression once per statement, instead of once per row.

Test case:

create sequence test_seq;

create table test_tab
( id number default  (more...)

What is overloading and how and when do I use it

Dear Patrick,

Recently I heard someone talk about overloading in Java. What is it, is it possible in PL/SQL and if so, how would I use it?

Ramesh Cumar

Dear Ramesh,

Overloading is a technique of creating multiple programs with the same name that can be called with different sets of parameters. It is definitely possible to apply this technique in PL/SQL, in fact, Oracle does this a lot of times in their own built-in (more...)

Celebrate Batman Day with SQL

Because today is Batman Day, it calls for a celebration of the SQL type:

SQL> select listagg (b)
2 within group (order by null) ||' Batman'
3 from (select 0f/0 b
4 from dual
5 connect by level <= 14
6 );

NanNanNanNanNanNanNanNanNanNanNanNanNanNan Batman

Oracle SQL Pattern Recognition – introducing the Match Recognize operator

imageOracle Database Release 12c (12.1) introduced a new operator that we can use in SQL queries. The MATCH_RECOGNIZE operator allows us to detect patterns in our relational data. Specifically: it allows us to identify records that mark the beginning of a set of records that together form a pattern. In this set, each record satisifies a certain condition. The fact that in a set of records these conditions are met – in a certain (more...)

When would you use a normal table function?

Dear Patrick,

Last year I did a presentation on table functions at KScope. One of the questions I got was: ‘If pipelined table functions provide their results faster, why would you want to use a normal table function?’ I couldn’t come up with the answer then, maybe you can help?

Erik van Roon

Dear Erik,

Let’s start with explaining a bit what table functions are. Table Functions are functions that return a collection of (more...)

Next Step in Row generation in Oracle Database 12c SQL using JSON_TABLE

One of those things SQL developers are frequently looking at is the generation of rows: having a query return records that do not really exist. For example to generate test data or to produce records for all days in a month. Tom Kyte usually selects from data dictionary views. Various tricks make the rounds, for example based on CONNECT BY or CUBE or UNPIVOT. This blog article by Natalka Roshak (2015) compares various row generation (more...)

What’s the difference between SEMI-JOIN and ANTI-JOIN?

Dear Patrick,

What is an ANTI-JOIN? And what is the difference between the SEMI-JOIN and the ANTI-JOIN?

Lillian Sturdey

Dear Lillian,

First of all, both SEMI-JOIN and ANTI-JOIN are not in the SQL syntax but they are more a pattern. You might expect to be able to write something like:

[PATRICK]SQL>SELECT d.deptno, d.dname, d.loc
               FROM dept d
               SEMI JOIN emp e ON (e.deptno = d.deptno)

to get all the (more...)

Find first n gaps in integer primary key


sokrates@12.1 > create table t( pk int primary key check(pk > 0));

Table created.

sokrates@12.1 > insert /*+ignore_row_on_dupkey_index(t(pk)) */ into t
sokrates@12.1 > select trunc(dbms_random.value(1, 1e5)) from dual
sokrates@12.1 > connect by level <= 1e5 
sokrates@12.1 > /

63187 rows created.

Finding the first n gaps

sokrates@12.1 > variable n number
sokrates@12.1 > exec :n := 1000

PL/SQL procedure successfully completed.

sokrates@12.1 > set autotr  (more...)

What is the difference between NVL and COALESCE?

Dear Patrick,

Could you tell me what the difference is between NVL and COALESCE?

Kindest regards,
Paul McCurdey

Dear Paul,

NVL returns the value of the first argument if it doesn’t evaluate to NULL, otherwise it will return the value of the second argument.
COALESCE returns the first argument that doesn’t evaluate to NULL. That can be any one of the arguments.
So they are definitely similar – but there are significant differences.
First of (more...)

How do I get my query results paginated?

Dear Patrick,

I have got a website with a search form. I want to display a limited number of results to the user and have him/her navigate through different pages. Is this possible using plain SQL?

Kindest regards,

Mitchell Ian

Dear Mitchell,

Of course this is possible. It might take some thinking, but that has never hurt anyone (yet). First we need a table with some randomly sorted data in it. In this example I (more...)

RegExp: Constraint to prevent spaces at the beginning or end.

Even though a space is a regular character, the client didn't want spaces at the beginning or end of a string. Any spaces in the middle were fine.
Of course this could be handled by the application, but it must also be implemented in the database. Using a check constraint with a regular expression will prevent the end user from entering unwanted data.

To try things out, let's just start with a simple table with (more...)

SUBSTR Versus LIKE in Oracle 11.2

I was reading an old SQL tuning book which was printed in 2002. It said that a where clause with like could often use an index whereas a similar clause using substr could not. I wondered if this might still be the case in an Oracle database. To find out, I created a table:

SQL> conn andrew/reid
SQL> create table tab1 as
  2  select table_name from dba_tables
  3  (more...)

Real World SQL and PL/SQL: Advice from the Experts

Because my hero is Cary Millsap, I'm going to do what he did and publish my foreword Preface. All joking aside, I consider myself incredibly fortunate to have been included in this project. I learned...a lot, by simply trying to find the author's mistakes (and there were not many). There was a lot more work than I expected, as well. (Technical) Editing is lot easier than writing, to be sure.

Brendan Tierney and (more...)

My first book is now physically in my hands.

Proud "parent" of a bouncing baby book

Proud “parent” of a bouncing baby book

Today a box arrived from Oracle Press. In it were a few copies of “Real-World SQL and PL/SQL” which I co-authored with Arup Nanda, Brendan Tierney, Heli Helskyaho and Alex Nuitjen. I know I only blogged about the book a couple of weeks back, how I became involved and the impact it had on my life for several months, but as I can now physically handle and read (more...)

The Book.

I’ve just added a picture to the right side of this site. It is for a book about SQL and PL/SQL. If you look at the image of the front cover, at the bottom is a list of authors and, near the end, is my name. It’s all finished and at the printers, but it is not out yet – It should be published in the next few weeks.

The British part of me wants (more...)

Splitting Strings: PL/SQL

Marc Bleron and Todd Hershiser gave me some very valuable feedback through their comments on my recent "Splitting Strings" posts. The big news is: PL/SQL beats SQL!