Weird SQL Results

Chris Taylor raised a good question.

Here is the interesting part (to me) – Sometimes it returns 1 row, sometimes it returns more than 1 row, and sometimes it returns no rows. To my mind, “trunc(dbms_random.value(1,500)” should return a random value between 1 & 500 every time and just one value.  Is this weird, or is it just me???

Basically he’s wondering why querying a table on a randomly-chosen unique identifier does not always return exactly 1 row.

At first glance it seems nonsensical and looks like a bug, but really, it’s not – in fact, Oracle is doing exactly the right thing (anecdotally, a colleague has told me that SQL Server gets this particular behaviour wrong).

A simple test case: the inner query generates exactly 100 rows, with the numbers 1 to 100. The outer query then queries this result set, with the intention of picking one of those rows only:

select * from (
  select rownum r from dual connect by level <= 100
where r = trunc(dbms_random.value(1,100));

However, what happens here is that Oracle knows that dbms_random.value is not deterministic; and the predicates (WHERE clause) must be evaluated once for each row returned from the FROM clause. Since dbms_random.value is not deterministic, Oracle knows it must re-evaluate it for each row, separately – which means it is comparing a different number for each row returned.

So, for example, Oracle looks at row 1, with the value 1. It generates a random number, e.g. 12, and so the predicate evaluates to FALSE, and the row is not returned. Oracle then looks at row 2, which has the value 2. It generates a random number, e.g. 2, and so the predicate evaluates to TRUE, and the row is returned. It does this for each row until the 100th is evaluated and then the query stops.

If none of the 100 random numbers happen to coincide with the values returned from the table, then the query will return no rows.

Quiz question: what is the theoretical maximum number of rows the above query could ever return?

To fix it so that Oracle only evaluates the selection criteria once, we just need to wrap it in another subquery:

select * from (
  select rownum r from dual connect by level <= 100
where r = (select trunc(dbms_random.value(1,100)) from dual);


  lexists      BOOLEAN;
  lfile_len    NUMBER;
  lblocksize   NUMBER;
    location    => 'a',
    filename    => 'b',
    exists      => lexists,
    file_length => lfile_len,
    blocksize   => lblocksize);

I was trying to use this procedure in a 9i database and kept getting:

PLS-00103: Encountered the symbol ">"...

– complaining about line 8 (the “exists” parameter). If I removed the parameter names, it worked fine. Something was wrong with that “exists” parameter name.
In the 9i and 10g documentation:

    location    IN VARCHAR2,
    filename    IN VARCHAR2,
    exists      OUT BOOLEAN,
    file_length OUT NUMBER,
    blocksize   OUT NUMBER);

In the 11g documentation:

    location    IN VARCHAR2,
    filename    IN VARCHAR2,
    fexists     OUT BOOLEAN,
    file_length OUT NUMBER,
    blocksize   OUT BINARY_INTEGER);

Ah – the parameter was actually called “fexists”. Ok. Try again:

PLS-00306: wrong number or types of arguments in call to 'FGETATTR'

Aaarrgh. Time for more googling.
According to psoug:

    location    IN  VARCHAR2,
    filename    IN  VARCHAR2,
    fexists     OUT BOOLEAN,
    file_length OUT NUMBER,
    block_size  OUT BINARY_INTEGER);

Thank goodness I’ve got access to more than just the Oracle docs!

A Good Bad Example

I learnt something new today (thanks to Steven) about TRIM – in the past I’ve used RTRIM and LTRIM if I only want to trim from the left or right ends of a string, but I was not aware (or had forgotten) that SQL’s TRIM function allows this to be specified.

Oracle’s documentation of TRIM, however, has a somewhat confusing example:

“This example trims leading zeros from the hire date of the employees in the hr schema:

SELECT employee_id,
FROM employees
WHERE department_id = 60
ORDER BY employee_id;

----------- ---------
103 3-JAN-90
104 21-MAY-91
105 25-JUN-97
106 5-FEB-98
107 7-FEB-99

As an example of using the TRIM function the above code is adequate; however, why does it call the TO_CHAR function at that point? After all, TO_CHAR is only useful if the datatype of its argument is not a character string – but in this case, it’s a string already – TRIM always returns a string.

Not only does the example perform a useless datatype conversion, it involves an implicit conversion – from date to string. The expression would be much better like this:


Better – but still not perfect. Sure, we should probably specify the date format so the code is a bit more robust (what if the session has a default format of ‘MM/DD/YY’?), but we can see yet another implicit data type conversion: “LEADING 0“. The only purpose for TRIM is to remove characters. Sure, you can put in zero if you want, but it’s only going to be converted to a character, ‘0’ anyway – so why not make it obvious to the next coder who comes along:


There, much better! I think the above expression is much clearer about what it will actually do – i.e. it converts a date to a string of characters, and then removes any leading ‘0’ characters.

The exception we never knew we needed

If, by some great random cosmic chance, you are a reader of this blog, but not of Tom Kyte‘s, then you would have missed this post:

NO_DATA_NEEDED – something I learned recently

It appears to have been documented in the 9i documentation, complete with spelling error:

ORA-06548, 00000, "no more rows needed"
Cause:   The caller of a pipelined function does not
         need more rows to be produced by the pipelined
Action:  Catch the NO_DATA_NEEDED exception is an
         exception handling block.

Mind you, it’s not all that obvious since if the pipelined function does not handle the exception, nothing goes wrong – the exception is never raised by the calling SQL statement. It’s not obvious when ORA-06548 would ever be raised.


Looks like ORA-06548 can appear in the error stack.

Customising a Navigation List

I like the “Vertical Images List” in APEX, which allows me to create a navigation bar of icons to give users quick access to various pages in my site. It’s easy to customise each item – you can select any image, add attributes for the image if necessary, and each item in the list has a URL which can point to another page in the application, or to an arbitrary URL.

My problem, however, was that some of the URLs in my list took the user to another site, or opened a PDF, and these would open in the same window. I wanted these particular items to open a new window, but the navigation item properties don’t allow this.

To solve this, I modified the Vertical Images List template, and used one of the User Defined Attributes to add “target=_blank” to the items that I wanted. While I was in there, I made a few tweaks to customise the template further to my liking.

A. Modify the Vertical Images List template.

  1. Go to Shared Components and open the Templates (under User Interface).
  2. Scroll down to Vertical Images List (in the Lists category) and open it for editing.
  3. Modify the Template Definition (WARNING: the code for different Apex templates may differ slightly; you’ll have to use a bit of nouse to customise it to your requirements) – you can add bits like #A01#, #A02#, etc – in my case I’ve used the following convention:

#A01# = extra text to appear below the icon & link;
#A02# = tooltip text for the hyperlink;
#A03# = extra attributes for the link (HTML <A> tag).
I’ve done this in both the “List Template Current” and “List Template Noncurrent” sections.

Customising the Vertical Image List template

For example, for “List Template Noncurrent, I’ve modified the template code as follows:

<tr><td><a href=”#LINK#” TITLE=”#A02#” #A03#>
<img src=”#IMAGE_PREFIX##IMAGE#” #IMAGE_ATTR# />#TEXT#</a>

B. Set User Defined Attributes.

  1. Open the Navigation List for editing (Shared Components -> Navigation -> Lists).
  2. Open the list item for editing that you wish to customise (or create a new one).
  3. In User Defined Attributes, attribute 1, add any text you wish to show beneath the link (but not highlighted as part of the ink)
  4. For attribute 2, add the title you wish to show up as a tooltip.
  5. For attribute 3, add the html attribute “target=_blank” if you wish this navigation entry to open a new window when invoked.

This is how it looks in a sample application:

If the user clicks on “Address Book”, the “target=_blank” attribute instructs the browser to open in a new window (or tab, in some cases).

Auto-print a page in APEX

My web site accepts applications for a sports team, and the last step is the applicant needs to print out a form to be signed. To make things as simple as possible, I want this form to send itself to their printer as soon as they open it. Now, I’m not a javascript expert; but instead of googling for the code, I stole it by doing a View Source on Google Mail’s print feature.

To get any APEX page to print when it is opened, all you need to do is add two bits to the page definition:

HTML Header

function Print(){document.body.offsetHeight;window.print()}

Page HTML Body Attribute


Isn’t javascript easy? I’m not sure what the “document.body.offsetHeight” is all about but I suspect it’s something to do with waiting for the whole page to load and render before the print starts.

“Not possible” is rarely correct

A good example of how not to answer a “Is it possible to…” question:

Is it possible to write a query which returns a date for every day between two specified days?

It’s ok to admit you don’t know how to do something.

It’s ok to say you don’t think a simple or feasible solution exists for a problem.

It may even be ok to say that something is impossible – if you constrain your answer to current technology. To say this you need to really know the technology, you need to have read about the problem widely enough, and you need to have enough personal experience to be able to say confidently, “no, what you are asking is impossible”. Even then, you might still be wrong, or become wrong sooner or later.

An answer saying outright, “No, that is impossible”, is just inviting a sharp rebuttal. Especially when in the very next sentence you admit that you aren’t an “Oracle specialist” 🙂

Truncated Mean in Oracle

A colleague needed to get the average from a set of data, but disregarding the top 25% and the bottom 25%. We didn’t know of any builtin Oracle function that would do this, but a review of the wikipedia page for Average yielded the technical term: truncated (or trimmed) mean. So we searched the Oracle docs and Google for ways to implement this function and didn’t come up with anything very useful. There were some SQL Server scripts which would have required two or three passes over the dataset.

After browsing through the aggregate functions documentation, I hit upon the NTILE function which I’ve used before, and realised that was the answer. The NTILE function takes a set of ordered data and divides it evenly (or as evenly as possible) between a number of buckets. In our case, we wanted to discard the top 25% and bottom 25%, so we simply divide the set into 4 buckets and discard the 1st and the 4th buckets; then take the standard average of the remainder:

SELECT AVG(mystat)
      SELECT mystat,
             NTILE(4) OVER (ORDER BY mystat) n
      FROM (SELECT mystat
            FROM mytable)
WHERE n IN (2,3);

The benefit of this query is that it only does one pass over the data, and was easily modified to partition the data set into groups; a count was also added so that the average could be taken over the entire data set for any groups that had less than 4 items.

To get the truncated mean in groups, except for groups with <4 items (for which we’ll report the average over the entire group):

SELECT mycat, AVG(mystat)
      SELECT mycat, mystat,
             NTILE(4) OVER (PARTITION BY mycat
                            ORDER BY mystat) n,
             COUNT(1) OVER (PARTITION BY mycat) c 
      FROM (SELECT mycat, mystat
            FROM mytable)
WHERE n IN (2,3) OR c < 4
GROUP BY mycat
ORDER BY mycat;