CLOB Concatenation

Adding string data to a CLOB in Oracle can be a tad slow, when compared to other string concatenation situations.  I first became aware of this concatenation performance issue from my own coding experience, but also from threads on Oracle Technical Network and a blog post by Jonathan Lewis.

Recursive Subquery Factoring Date Bug in 11gR2

Since Oracle 11gR2, Oracle has supported Recursive Subquery Factoring – this is a more flexible way of doing recursive queries than Oracle’s Connect By. Although 11g is quite old now, it’s still being used in earnest.  Whilst writing some code recently, I noticed there appears to be a problem with them regarding dates. I found this in version 11.2.0.2.0. Let’s take a look at what I found…

with t1(a_date) as (
  select date '2018-12-26'
  from   dual
  union all
  select a_date + 1 -- Problem here
  from   t1
  where a_date + 1 < date '2018-12-30'
)
select * from t1

Error at line 5
ORA-01790: expression must have same datatype as corresponding expression

Faster Decimal to Binary Function

There was recently a post on the Oracle forums by a person who needed to convert decimal values to a binary string. They had found the standard library PL/SQL function that you often see on Oracle sites, but its slow performance was giving him headaches. It was an interesting thread as many people offered solutions, and it showed the ingenuity of people, in coming up with alternatives. In this article I’ll show some of my efforts, and my final function which, to my pleasure, was the fastest in the thread.

Inheriting Values in SQL

Imagine a situation where you have hierarchical data with various values and you wanted to inherit values from ancestors if a given level in the hierarchy had null values. So if a level had a null value, it could inherit that missing value from the parent and if the parent didn’t have a value, the grandparent etc. etc. updwards until we reach the root. How would you do it?  Well in this article I’ll explore a few approaches.

Parameterised Views

Background

Occasionally I see posts on Oracle sites from users asking how to create a view based on parameter(s).  Now, many would ask why would you’d need this, especially when you can simply select from the view and apply a where clause?  Well, there are a few situations which fall outside of that approach, the commonest it probably that the view contains a function that needs parameters or the view requires some “state” in its operation (state means there is memory of the past).  Anyway, assuming you have such a need let’s look at some of the approaches at our disposal.

How to Convert Accented Characters into Non Accented Ones

A while ago a user on an Oracle forum I post to asked how he could convert accented characters into non accented characters (note, he didn’t care about case sensitivity, just wanted rid of the accents).  This was because he was creating a database role from this via an Web API and Oracle was raising an ORA-00911 invalid character whenever an API call was made with an accented character.

I knew that when Oracle did binary sorts on strings, it needed to forget about accents, meaning it had such functionality built in somewhere.  So the solution I gave him ended up leveraging this.

Anyway, here’s an example of the solution using a full list of European accented characters.

with t as (
  select 'ÂÃÄÀÁÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõöøùúûüýþÿ' str 
  from dual
)
select str, utl_raw.cast_to_varchar2(nlssort(str, 'nls_sort=binary_ai')) str2 
from t

STR                                                            STR2 
-------------------------------------------------------------- ---------------------------------------------------------------
ÂÃÄÀÁÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõöøùúûüýþÿ aaaaaaæceeeeiiiiðnoooooouuuuyþssaaaaaaæceeeeiiiiðnoooooouuuuyþy 
1 row selected.

I use Oracle’s inbuilt nlssort functionality which converts a string into a raw (string of bytes) used for sorting, and I simply specify that raw is to be accent and case insensitive. I then simply cast that raw back into a varchar2 – which results in replacing accented chars with their non accented, non case sensitive equivalent!  If he wanted this uppercase, he could simply UPPER(..) it.

Note : Some special characters in Icelandic / Turkish have no unaccented equivalent, but then there’s nothing we can do about those.  

NO_DATA_FOUND when using aggregate functions like count, min, max, avg…

A common misconception when writing PL/SQL select …. into … statements is that if it is an aggregation query (a query that is using an aggregation like count, max, min, avg, etc.) then they always return a result – possibly null – so a NO_DATA_FOUND exception will never occur.

During a post on the Oracle Community Forums, a regular and respected contributor made a statement to this effect, before the classic error was pointed out to him.

Let’s take a look at when, how and why NO_DATA_FOUND exceptions occur and show that they can indeed occur when using aggregated functions.